diff --git a/BUILD.gn b/BUILD.gn
index 9cc4500..ebe6c0d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -796,6 +796,8 @@
   sources = [
     "core/fxcrt/cfx_bytestring.cpp",
     "core/fxcrt/cfx_bytestring.h",
+    "core/fxcrt/cfx_datetime.cpp",
+    "core/fxcrt/cfx_datetime.h",
     "core/fxcrt/cfx_maybe_owned.h",
     "core/fxcrt/cfx_memorystream.cpp",
     "core/fxcrt/cfx_memorystream.h",
@@ -874,8 +876,6 @@
       "core/fxcrt/cfx_checksumcontext.h",
       "core/fxcrt/cfx_crtfileaccess.cpp",
       "core/fxcrt/cfx_crtfileaccess.h",
-      "core/fxcrt/cfx_datetime.cpp",
-      "core/fxcrt/cfx_datetime.h",
       "core/fxcrt/cfx_decimal.cpp",
       "core/fxcrt/cfx_decimal.h",
       "core/fxcrt/cfx_seekablestreamproxy.cpp",
diff --git a/core/fxcrt/cfx_datetime.cpp b/core/fxcrt/cfx_datetime.cpp
index 833fe58..b59aabc 100644
--- a/core/fxcrt/cfx_datetime.cpp
+++ b/core/fxcrt/cfx_datetime.cpp
@@ -109,7 +109,7 @@
   utLocal.wMinute = st.tm_min;
   utLocal.wSecond = st.tm_sec;
   utLocal.wMillisecond = curTime.tv_usec / 1000;
-#endif  // _FX_OS_ == _FX_WIN32_DESKTOP_ || _FX_OS_ == _FX_WIN32_MOBILE_ || \
+#endif  // _FX_OS_ == _FX_WIN32_DESKTOP_ || _FX_OS_ == _FX_WIN32_MOBILE_ ||
         // _FX_OS_ == _FX_WIN64_
 
   year_ = utLocal.wYear;
diff --git a/fpdfsdk/fpdfattachment.cpp b/fpdfsdk/fpdfattachment.cpp
index 7d8cce7..724664b 100644
--- a/fpdfsdk/fpdfattachment.cpp
+++ b/fpdfsdk/fpdfattachment.cpp
@@ -4,14 +4,45 @@
 
 #include "public/fpdf_attachment.h"
 
+#include "core/fdrm/crypto/fx_crypt.h"
 #include "core/fpdfapi/page/cpdf_streamparser.h"
+#include "core/fpdfapi/parser/cpdf_array.h"
 #include "core/fpdfapi/parser/cpdf_document.h"
+#include "core/fpdfapi/parser/cpdf_name.h"
+#include "core/fpdfapi/parser/cpdf_number.h"
+#include "core/fpdfapi/parser/cpdf_reference.h"
 #include "core/fpdfapi/parser/cpdf_string.h"
 #include "core/fpdfapi/parser/fpdf_parser_decode.h"
 #include "core/fpdfdoc/cpdf_filespec.h"
 #include "core/fpdfdoc/cpdf_nametree.h"
+#include "core/fxcrt/cfx_datetime.h"
+#include "core/fxcrt/fx_extension.h"
 #include "fpdfsdk/fsdk_define.h"
 
+namespace {
+
+CFX_ByteString CFXByteStringHexDecode(const CFX_ByteString& bsHex) {
+  uint8_t* result = nullptr;
+  uint32_t size = 0;
+  HexDecode(bsHex.raw_str(), bsHex.GetLength(), &result, &size);
+  CFX_ByteString bsDecoded(result, size);
+  FX_Free(result);
+  return bsDecoded;
+}
+
+CFX_ByteString GenerateMD5Base16(const void* contents,
+                                 const unsigned long len) {
+  uint8_t digest[16];
+  CRYPT_MD5Generate(reinterpret_cast<const uint8_t*>(contents), len, digest);
+  char buf[32];
+  for (int i = 0; i < 16; ++i)
+    FXSYS_IntToTwoHexChars(digest[i], &buf[i * 2]);
+
+  return CFX_ByteString(buf, 32);
+}
+
+}  // namespace
+
 DLLEXPORT int STDCALL FPDFDoc_GetAttachmentCount(FPDF_DOCUMENT document) {
   CPDF_Document* pDoc = CPDFDocumentFromFPDFDocument(document);
   if (!pDoc)
@@ -20,6 +51,50 @@
   return CPDF_NameTree(pDoc, "EmbeddedFiles").GetCount();
 }
 
+DLLEXPORT FPDF_ATTACHMENT FPDFDoc_AddAttachment(FPDF_DOCUMENT document,
+                                                FPDF_WIDESTRING name) {
+  CPDF_Document* pDoc = CPDFDocumentFromFPDFDocument(document);
+  CFX_WideString wsName =
+      CFX_WideString::FromUTF16LE(name, CFX_WideString::WStringLength(name));
+  if (!pDoc || wsName.IsEmpty())
+    return nullptr;
+
+  CPDF_Dictionary* pRoot = pDoc->GetRoot();
+  if (!pRoot)
+    return nullptr;
+
+  // Retrieve the document's Names dictionary; create it if missing.
+  CPDF_Dictionary* pNames = pRoot->GetDictFor("Names");
+  if (!pNames) {
+    pNames = pDoc->NewIndirect<CPDF_Dictionary>();
+    pRoot->SetNewFor<CPDF_Reference>("Names", pDoc, pNames->GetObjNum());
+  }
+
+  // Create the EmbeddedFiles dictionary if missing.
+  if (!pNames->GetDictFor("EmbeddedFiles")) {
+    CPDF_Dictionary* pFiles = pDoc->NewIndirect<CPDF_Dictionary>();
+    pFiles->SetNewFor<CPDF_Array>("Names");
+    pNames->SetNewFor<CPDF_Reference>("EmbeddedFiles", pDoc,
+                                      pFiles->GetObjNum());
+  }
+
+  // Set up the basic entries in the filespec dictionary.
+  CPDF_Dictionary* pFile = pDoc->NewIndirect<CPDF_Dictionary>();
+  pFile->SetNewFor<CPDF_Name>("Type", "Filespec");
+  pFile->SetNewFor<CPDF_String>("UF", wsName);
+  pFile->SetNewFor<CPDF_String>("F", wsName);
+
+  // Add the new attachment name and filespec into the document's EmbeddedFiles.
+  CPDF_NameTree nameTree(pDoc, "EmbeddedFiles");
+  if (!nameTree.AddValueAndName(
+          pdfium::MakeUnique<CPDF_Reference>(pDoc, pFile->GetObjNum()),
+          wsName)) {
+    return nullptr;
+  }
+
+  return pFile;
+}
+
 DLLEXPORT FPDF_ATTACHMENT STDCALL FPDFDoc_GetAttachment(FPDF_DOCUMENT document,
                                                         int index) {
   CPDF_Document* pDoc = CPDFDocumentFromFPDFDocument(document);
@@ -73,6 +148,28 @@
   return pObj->GetType();
 }
 
+DLLEXPORT FPDF_BOOL STDCALL
+FPDFAttachment_SetStringValue(FPDF_ATTACHMENT attachment,
+                              FPDF_WIDESTRING key,
+                              FPDF_WIDESTRING value) {
+  CPDF_Object* pFile = CPDFObjectFromFPDFAttachment(attachment);
+  if (!pFile)
+    return false;
+
+  CPDF_Dictionary* pParamsDict = CPDF_FileSpec(pFile).GetParamsDict();
+  if (!pParamsDict)
+    return false;
+
+  CFX_ByteString bsKey = CFXByteStringFromFPDFWideString(key);
+  CFX_ByteString bsValue = CFXByteStringFromFPDFWideString(value);
+  bool bEncodedAsHex = bsKey == "CheckSum";
+  if (bEncodedAsHex)
+    bsValue = CFXByteStringHexDecode(bsValue);
+
+  pParamsDict->SetNewFor<CPDF_String>(bsKey, bsValue, bEncodedAsHex);
+  return true;
+}
+
 DLLEXPORT unsigned long STDCALL
 FPDFAttachment_GetStringValue(FPDF_ATTACHMENT attachment,
                               FPDF_WIDESTRING key,
@@ -88,7 +185,7 @@
 
   CFX_ByteString bsKey = CFXByteStringFromFPDFWideString(key);
   CFX_WideString value = pParamsDict->GetUnicodeTextFor(bsKey);
-  if (bsKey == "CheckSum") {
+  if (bsKey == "CheckSum" && !value.IsEmpty()) {
     CPDF_String* stringValue = pParamsDict->GetObjectFor(bsKey)->AsString();
     if (stringValue->IsHex()) {
       value =
@@ -101,6 +198,53 @@
   return Utf16EncodeMaybeCopyAndReturnLength(value, buffer, buflen);
 }
 
+DLLEXPORT FPDF_BOOL FPDFAttachment_SetFile(FPDF_ATTACHMENT attachment,
+                                           FPDF_DOCUMENT document,
+                                           const void* contents,
+                                           const unsigned long len) {
+  CPDF_Object* pFile = CPDFObjectFromFPDFAttachment(attachment);
+  CPDF_Document* pDoc = CPDFDocumentFromFPDFDocument(document);
+  if (!pFile || !pFile->IsDictionary() || !pDoc || len > INT_MAX)
+    return false;
+
+  // An empty content must have a zero length.
+  if (!contents && len != 0)
+    return false;
+
+  // Create a dictionary for the new embedded file stream.
+  auto pFileStreamDict = pdfium::MakeUnique<CPDF_Dictionary>();
+  CPDF_Dictionary* pParamsDict =
+      pFileStreamDict->SetNewFor<CPDF_Dictionary>("Params");
+
+  // Set the size of the new file in the dictionary.
+  pFileStreamDict->SetNewFor<CPDF_Number>("DL", static_cast<int>(len));
+  pParamsDict->SetNewFor<CPDF_Number>("Size", static_cast<int>(len));
+
+  // Set the creation date of the new attachment in the dictionary.
+  CFX_DateTime dateTime;
+  dateTime.Now();
+  CFX_ByteString bsDateTime;
+  bsDateTime.Format("D:%d%02d%02d%02d%02d%02d", dateTime.GetYear(),
+                    dateTime.GetMonth(), dateTime.GetDay(), dateTime.GetHour(),
+                    dateTime.GetMinute(), dateTime.GetSecond());
+  pParamsDict->SetNewFor<CPDF_String>("CreationDate", bsDateTime, false);
+
+  // Set the checksum of the new attachment in the dictionary.
+  pParamsDict->SetNewFor<CPDF_String>(
+      "CheckSum", CFXByteStringHexDecode(GenerateMD5Base16(contents, len)),
+      true);
+
+  // Create the file stream and have the filespec dictionary link to it.
+  std::unique_ptr<uint8_t, FxFreeDeleter> stream(FX_Alloc(uint8_t, len));
+  memcpy(stream.get(), contents, len);
+  CPDF_Stream* pFileStream = pDoc->NewIndirect<CPDF_Stream>(
+      std::move(stream), len, std::move(pFileStreamDict));
+  CPDF_Dictionary* pEFDict =
+      pFile->AsDictionary()->SetNewFor<CPDF_Dictionary>("EF");
+  pEFDict->SetNewFor<CPDF_Reference>("F", pDoc, pFileStream->GetObjNum());
+  return true;
+}
+
 DLLEXPORT unsigned long STDCALL
 FPDFAttachment_GetFile(FPDF_ATTACHMENT attachment,
                        void* buffer,
diff --git a/fpdfsdk/fpdfattachment_embeddertest.cpp b/fpdfsdk/fpdfattachment_embeddertest.cpp
index d873d9b..f4d0bfc 100644
--- a/fpdfsdk/fpdfattachment_embeddertest.cpp
+++ b/fpdfsdk/fpdfattachment_embeddertest.cpp
@@ -85,3 +85,153 @@
   EXPECT_EQ(kCheckSumW,
             GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data())));
 }
+
+TEST_F(FPDFAttachmentEmbeddertest, AddAttachments) {
+  // Open a file with two attachments.
+  ASSERT_TRUE(OpenDocument("embedded_attachments.pdf"));
+  EXPECT_EQ(2, FPDFDoc_GetAttachmentCount(document()));
+
+  // Check that adding an attachment with an empty name would fail.
+  EXPECT_FALSE(FPDFDoc_AddAttachment(document(), nullptr));
+
+  // Add an attachment to the beginning of the embedded file list.
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> file_name =
+      GetFPDFWideString(L"0.txt");
+  FPDF_ATTACHMENT attachment =
+      FPDFDoc_AddAttachment(document(), file_name.get());
+
+  // Check that writing to a file with nullptr but non-zero bytes would fail.
+  EXPECT_FALSE(FPDFAttachment_SetFile(attachment, document(), nullptr, 10));
+
+  // Set the new attachment's file.
+  constexpr char kContents1[] = "Hello!";
+  EXPECT_TRUE(FPDFAttachment_SetFile(attachment, document(), kContents1,
+                                     strlen(kContents1)));
+
+  // Verify the name of the new attachment (i.e. the first attachment).
+  attachment = FPDFDoc_GetAttachment(document(), 0);
+  ASSERT_TRUE(attachment);
+  unsigned long len = FPDFAttachment_GetName(attachment, nullptr, 0);
+  std::vector<char> buf(len);
+  EXPECT_EQ(12u, FPDFAttachment_GetName(attachment, buf.data(), len));
+  EXPECT_STREQ(L"0.txt",
+               GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data()))
+                   .c_str());
+
+  // Verify the content of the new attachment (i.e. the first attachment).
+  len = FPDFAttachment_GetFile(attachment, nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  ASSERT_EQ(6u, FPDFAttachment_GetFile(attachment, buf.data(), len));
+  EXPECT_EQ(std::string(kContents1), std::string(buf.data(), 6));
+
+  // Add an attachment to the end of the embedded file list and set its file.
+  file_name = GetFPDFWideString(L"z.txt");
+  attachment = FPDFDoc_AddAttachment(document(), file_name.get());
+  constexpr char kContents2[] = "World!";
+  EXPECT_TRUE(FPDFAttachment_SetFile(attachment, document(), kContents2,
+                                     strlen(kContents2)));
+  EXPECT_EQ(4, FPDFDoc_GetAttachmentCount(document()));
+
+  // Verify the name of the new attachment (i.e. the fourth attachment).
+  attachment = FPDFDoc_GetAttachment(document(), 3);
+  ASSERT_TRUE(attachment);
+  len = FPDFAttachment_GetName(attachment, nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  EXPECT_EQ(12u, FPDFAttachment_GetName(attachment, buf.data(), len));
+  EXPECT_STREQ(L"z.txt",
+               GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data()))
+                   .c_str());
+
+  // Verify the content of the new attachment (i.e. the fourth attachment).
+  len = FPDFAttachment_GetFile(attachment, nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  ASSERT_EQ(6u, FPDFAttachment_GetFile(attachment, buf.data(), len));
+  EXPECT_EQ(std::string(kContents2), std::string(buf.data(), 6));
+}
+
+TEST_F(FPDFAttachmentEmbeddertest, AddAttachmentsWithParams) {
+  // Open a file with two attachments.
+  ASSERT_TRUE(OpenDocument("embedded_attachments.pdf"));
+  EXPECT_EQ(2, FPDFDoc_GetAttachmentCount(document()));
+
+  // Add an attachment to the embedded file list.
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> file_name =
+      GetFPDFWideString(L"5.txt");
+  FPDF_ATTACHMENT attachment =
+      FPDFDoc_AddAttachment(document(), file_name.get());
+  constexpr char kContents[] = "Hello World!";
+  EXPECT_TRUE(FPDFAttachment_SetFile(attachment, document(), kContents,
+                                     strlen(kContents)));
+
+  // Set the date to be an arbitrary value.
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> date_key =
+      GetFPDFWideString(L"CreationDate");
+  constexpr wchar_t kDateW[] = L"D:20170720161527-04'00'";
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> ws_date =
+      GetFPDFWideString(kDateW);
+  EXPECT_TRUE(
+      FPDFAttachment_SetStringValue(attachment, date_key.get(), ws_date.get()));
+
+  // Set the checksum to be an arbitrary value.
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> checksum_key =
+      GetFPDFWideString(L"CheckSum");
+  constexpr wchar_t kCheckSumW[] = L"<ABCDEF01234567899876543210FEDCBA>";
+  std::unique_ptr<unsigned short, pdfium::FreeDeleter> ws_checksum =
+      GetFPDFWideString(kCheckSumW);
+  EXPECT_TRUE(FPDFAttachment_SetStringValue(attachment, checksum_key.get(),
+                                            ws_checksum.get()));
+
+  // Verify the name of the new attachment (i.e. the second attachment).
+  attachment = FPDFDoc_GetAttachment(document(), 1);
+  ASSERT_TRUE(attachment);
+  unsigned long len = FPDFAttachment_GetName(attachment, nullptr, 0);
+  std::vector<char> buf(len);
+  EXPECT_EQ(12u, FPDFAttachment_GetName(attachment, buf.data(), len));
+  EXPECT_STREQ(L"5.txt",
+               GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data()))
+                   .c_str());
+
+  // Verify the content of the new attachment.
+  len = FPDFAttachment_GetFile(attachment, nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  ASSERT_EQ(12u, FPDFAttachment_GetFile(attachment, buf.data(), len));
+  EXPECT_EQ(std::string(kContents), std::string(buf.data(), 12));
+
+  // Verify the creation date of the new attachment.
+  len = FPDFAttachment_GetStringValue(attachment, date_key.get(), nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  EXPECT_EQ(48u, FPDFAttachment_GetStringValue(attachment, date_key.get(),
+                                               buf.data(), len));
+  EXPECT_STREQ(kDateW,
+               GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data()))
+                   .c_str());
+
+  // Verify the checksum of the new attachment.
+  len =
+      FPDFAttachment_GetStringValue(attachment, checksum_key.get(), nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  EXPECT_EQ(70u, FPDFAttachment_GetStringValue(attachment, checksum_key.get(),
+                                               buf.data(), len));
+  EXPECT_STREQ(kCheckSumW,
+               GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data()))
+                   .c_str());
+
+  // Overwrite the existing file with empty content, and check that the checksum
+  // gets updated to the correct value.
+  EXPECT_TRUE(FPDFAttachment_SetFile(attachment, document(), nullptr, 0));
+  EXPECT_EQ(0u, FPDFAttachment_GetFile(attachment, nullptr, 0));
+  len =
+      FPDFAttachment_GetStringValue(attachment, checksum_key.get(), nullptr, 0);
+  buf.clear();
+  buf.resize(len);
+  EXPECT_EQ(70u, FPDFAttachment_GetStringValue(attachment, checksum_key.get(),
+                                               buf.data(), len));
+  EXPECT_EQ(L"<D41D8CD98F00B204E9800998ECF8427E>",
+            GetPlatformWString(reinterpret_cast<unsigned short*>(buf.data())));
+}
\ No newline at end of file
diff --git a/fpdfsdk/fpdfview_c_api_test.c b/fpdfsdk/fpdfview_c_api_test.c
index 97bacbb..95a0175 100644
--- a/fpdfsdk/fpdfview_c_api_test.c
+++ b/fpdfsdk/fpdfview_c_api_test.c
@@ -66,11 +66,14 @@
 
     // fpdf_attachment.h
     CHK(FPDFDoc_GetAttachmentCount);
+    CHK(FPDFDoc_AddAttachment);
     CHK(FPDFDoc_GetAttachment);
     CHK(FPDFAttachment_GetName);
     CHK(FPDFAttachment_HasKey);
     CHK(FPDFAttachment_GetValueType);
+    CHK(FPDFAttachment_SetStringValue);
     CHK(FPDFAttachment_GetStringValue);
+    CHK(FPDFAttachment_SetFile);
     CHK(FPDFAttachment_GetFile);
 
     // fpdf_dataavail.h
diff --git a/public/fpdf_attachment.h b/public/fpdf_attachment.h
index 2c40992..6a23970 100644
--- a/public/fpdf_attachment.h
+++ b/public/fpdf_attachment.h
@@ -21,6 +21,19 @@
 DLLEXPORT int STDCALL FPDFDoc_GetAttachmentCount(FPDF_DOCUMENT document);
 
 // Experimental API.
+// Add an embedded file with |name| in |document|. If |name| is empty, or if
+// |name| is the name of a existing embedded file in |document|, or if
+// |document|'s embedded file name tree is too deep (i.e. |document| has too
+// many embedded files already), then a new attachment will not be added.
+//
+//   document - handle to a document.
+//   name     - name of the new attachment.
+//
+// Returns a handle to the new attachment object, or NULL on failure.
+DLLEXPORT FPDF_ATTACHMENT FPDFDoc_AddAttachment(FPDF_DOCUMENT document,
+                                                FPDF_WIDESTRING name);
+
+// Experimental API.
 // Get the embedded attachment at |index| in |document|. Note that the returned
 // attachment handle is only valid while |document| is open.
 //
@@ -68,6 +81,21 @@
 FPDFAttachment_GetValueType(FPDF_ATTACHMENT attachment, FPDF_WIDESTRING key);
 
 // Experimental API.
+// Set the string value corresponding to |key| in the params dictionary of the
+// embedded file |attachment|, overwriting the existing value if any. The value
+// type should be FPDF_OBJECT_STRING after this function call succeeds.
+//
+//   attachment - handle to an attachment.
+//   key        - the key to the dictionary entry, encoded in UTF16-LE.
+//   value      - the string value to be set, encoded in UTF16-LE.
+//
+// Returns true if successful.
+DLLEXPORT FPDF_BOOL STDCALL
+FPDFAttachment_SetStringValue(FPDF_ATTACHMENT attachment,
+                              FPDF_WIDESTRING key,
+                              FPDF_WIDESTRING value);
+
+// Experimental API.
 // Get the string value corresponding to |key| in the params dictionary of the
 // embedded file |attachment|. |buffer| is only modified if |buflen| is longer
 // than the length of the string value. Note that if |key| does not exist in the
@@ -79,7 +107,7 @@
 //
 //   attachment - handle to an attachment.
 //   key        - the key to the requested string value.
-//   buffer     - buffer for holding the file's date string encoded in UTF16-LE.
+//   buffer     - buffer for holding the string value encoded in UTF16-LE.
 //   buflen     - length of the buffer.
 //
 // Returns the length of the dictionary value string.
@@ -90,12 +118,28 @@
                               unsigned long buflen);
 
 // Experimental API.
+// Set the file data of |attachment|, overwriting the existing file data if any.
+// The creation date and checksum will be updated, while all other dictionary
+// entries will be deleted. Note that only contents with |len| smaller than
+// INT_MAX is supported.
+//
+//   attachment - handle to an attachment.
+//   contents   - buffer holding the file data to be written in raw bytes.
+//   len        - length of file data.
+//
+// Returns true if successful.
+DLLEXPORT FPDF_BOOL FPDFAttachment_SetFile(FPDF_ATTACHMENT attachment,
+                                           FPDF_DOCUMENT document,
+                                           const void* contents,
+                                           const unsigned long len);
+
+// Experimental API.
 // Get the file data of |attachment|. |buffer| is only modified if |buflen| is
 // longer than the length of the file. On errors, |buffer| is unmodified and the
 // returned length is 0.
 //
 //   attachment - handle to an attachment.
-//   buffer     - buffer for holding the file's data in raw bytes.
+//   buffer     - buffer for holding the file data in raw bytes.
 //   buflen     - length of the buffer.
 //
 // Returns the length of the file.