I found in a obscure API document from OpenText that you could set the property FILE_EXTENSION. I got the same issue and it fixed it. So in your case it should be

  $doc.SetProperty("FILE_EXTENSION", $appid)

Though you mention you tried it, it worked in my case.

The short answer is that you cannot change the file extension.

When you upload a file, the fileextension is uses to choose the application on the profile. In DM531/DM10 each application can have one default extension (DOCX or DOC for word), then you have to log on to the CyberDocs to add other file extensions. The extensions are added to the table APP_FILE_EXTNS note that you cannot add rows since the system_id is generated by the eDOCS API. The system_id is created using the sequence tables SEQ_xxx. The applications is in the APPS table.

So you have to ask your eDOCS DM admin to add the needed file extensions using the Library Maintenance or the DM Management Studio tool (DM10 or DM 16.x).

I am not sure if this will be helpful but I have found this code below where the extension is set.

            Function ahtCommonFileOpenSave( _
            Optional ByRef flags As Variant, _
            Optional ByVal InitialDir As Variant, _
            Optional ByVal Filter As Variant, _
            Optional ByVal FilterIndex As Variant, _
            Optional ByVal DefaultExt As Variant, _
            Optional ByVal FileName As Variant, _
            Optional ByVal DialogTitle As Variant, _
            Optional ByVal hwnd As Variant, _
            Optional ByVal OpenFile As Variant) As Variant

                ' This is the entry point you'll use to call the common
                ' file open/save dialog. The parameters are listed
                ' below, and all are optional.
                '
                ' In:
                ' Flags: one or more of the ahtOFN_* constants, OR'd together.
                ' InitialDir: the directory in which to first look
                ' Filter: a set of file filters, set up by calling
                ' AddFilterItem. See examples.
                ' FilterIndex: 1-based integer indicating which filter
                ' set to use, by default (1 if unspecified)
                ' DefaultExt: Extension to use if the user doesn't enter one.
                ' Only useful on file saves.
                ' FileName: Default value for the file name text box.
                ' DialogTitle: Title for the dialog.
                ' hWnd: parent window handle
                ' OpenFile: Boolean(True=Open File/False=Save As)
                ' Out:
                ' Return Value: Either Null or the selected filename

                Dim OFN As tagOPENFILENAME
                Dim strFilename As String
                Dim strFileTitle As String
                Dim fResult As Boolean

                ' Give the dialog a caption title.
                If IsMissing(InitialDir) Then InitialDir = CurDir
                If IsMissing(Filter) Then Filter = ""
                If IsMissing(FilterIndex) Then FilterIndex = 1
                If IsMissing(flags) Then flags = 0&
                If IsMissing(DefaultExt) Then DefaultExt = ""
                If IsMissing(FileName) Then FileName = ""
                If IsMissing(DialogTitle) Then DialogTitle = ""
                If IsMissing(OpenFile) Then OpenFile = True

                ' Allocate string space for the returned strings.
                strFilename = Left(FileName & String(256, 0), 256)
                strFileTitle = String(256, 0)

                ' Set up the data structure before you call the function
                With OFN
                    .lStructSize = Len(OFN)
                    .hwndOwner = hwnd
                    .strFilter = Filter
                    .nFilterIndex = FilterIndex
                    .strFile = strFilename
                    .nMaxFile = Len(strFilename)
                    .strFileTitle = strFileTitle
                    .nMaxFileTitle = Len(strFileTitle)
                    .strTitle = DialogTitle
                    .flags = flags
                    .strDefExt = DefaultExt
                    .strInitialDir = InitialDir
                    ' Didn't think most people would want to deal with
                    ' these options.
                    .hInstance = 0
                    .strCustomFilter = ""
                    .nMaxCustFilter = 0
                    .lpfnHook = 0
                    'New for NT 4.0
                    .strCustomFilter = String(255, 0)
                    .nMaxCustFilter = 255
                End With

                ' This will pass the desired data structure to the
                ' Windows API, which will in turn it uses to display
                ' the Open/Save As Dialog.

                If OpenFile Then

                    fResult = aht_apiGetOpenFileName(OFN)

                Else

                    fResult = aht_apiGetSaveFileName(OFN)

                End If

                ' The function call filled in the strFileTitle member
                ' of the structure. You'll have to write special code
                ' to retrieve that if you're interested.

                If fResult Then
                    ' You might care to check the Flags member of the
                    ' structure to get information about the chosen file.
                    ' In this example, if you bothered to pass in a
                    ' value for Flags, we'll fill it in with the outgoing
                    ' Flags value.
                    If Not IsMissing(flags) Then
                        flags = OFN.flags
                    End If

                    ahtCommonFileOpenSave = TrimNull(OFN.strFile)

                Else

                    ahtCommonFileOpenSave = vbNullString

                End If

            End Function