質問のリンクにあるコード サンプルを使用して、リクエストを手動で作成するように変更したところ、機能しました。また、バイナリ ファイルと 1 つのパラメーターを受け取り、要求を POST する単一の関数に変換しました。
Function PostDocument(intDocumentID, binFile, strFilename, strContentType)
Dim objHttp, strBoundary, strRequestStart, strRequestEnd, binPost
Dim objStream
strBoundary = "---------------------------9849436581144108930470211272"
Set objHttp = Server.CreateObject("MSXML2.ServerXMLHTTP.6.0")
strRequestStart = "--" & strBoundary & vbCrlf &_
"Content-Disposition: form-data; name=""id""" & vbCrlf &_
vbCrlf &_
intDocumentID & vbCrlf &_
vbCrlf &_
"--" & strBoundary & vbCrlf &_
"Content-Disposition: form-data; name=""file""; filename=""" & strFilename & """" & vbCrlf &_
"Content-Type: " & strContentType & vbCrlf &_
vbCrlf
strRequestEnd = vbCrLf & "--" & strBoundary & "--"
Set objStream = Server.CreateObject("ADODB.Stream")
objStream.Type = adTypeBinary '1
objStream.Mode = adModeReadWrite '3
objStream.Open
objStream.Write StringToBinary(strRequestStart)
objStream.Write binFile
objStream.Write StringToBinary(strRequestEnd)
objStream.Position = 0
binPost = objStream.Read
Response.Write binPost
objStream.Close
Set objStream = Nothing
objHttp.Open "POST", "(url removed)", False, "(username removed)", "(password removed)"
objHttp.setRequestHeader "Content-Type", "multipart/form-data; boundary=""" & strBoundary & """"
objHttp.Send binPost
PostDocument = objHttp.ResponseText
Set objHttp = Nothing
End Function
Function StringToBinary(toConvert)
Dim objStream, data
Set objStream = Server.CreateObject("ADODB.Stream")
objStream.Charset = "ISO-8859-1"
objStream.Type = adTypeText '2
objStream.Mode = adModeReadWrite '3
objStream.Open
objStream.WriteText toConvert
objStream.Position = 0
objStream.Type = adTypeBinary '1
StringToBinary = objStream.Read
objStream.Close
Set objStream = Nothing
End Function