Unfortunately, in Extensions it becomes fiddly when it is necessary to upload or download File Attachments. The build-in functions of Codeunit “File Mgt.” and Table “Attachment” are not allowed to be used in Extension development. Therefor it is necessary to create own functions. The code below works in every browser.

Upload File Attachment

procedure UploadAttachment()
var
    AttachmentRec: Record Attachment;
    FileOutStream: OutStream;
    FileInStream: InStream;
    tempfilename: text;
    DialogTitle: Label 'Please select a File...';
begin
    if UploadIntoStream(DialogTitle, '', 'All Files (*.*)|*.*', tempfilename, FileInStream) then begin

        AttachmentRec.Init();
        AttachmentRec.Insert(true);
        AttachmentRec."Storage Type" := AttachmentRec."Storage Type"::Embedded;
        AttachmentRec."Storage Pointer" := '';
        AttachmentRec."File Extension" := GetFileType(tempfilename);
        AttachmentRec."Attachment File".CreateOutStream(FileOutStream);
        CopyStream(FileOutStream, FileInStream);
        AttachmentRec.Modify(true);

    end;
end;

local procedure GetFileType(pFilename: Text): Text;
var
    FilenamePos: Integer;
begin
    FilenamePos := StrLen(pFilename);
    while (pFilename[FilenamePos] <> '.') or (FilenamePos < 1) do
        FilenamePos -= 1;

    if FilenamePos = 0 then
        exit('');

    exit(CopyStr(pFilename, FilenamePos + 1, StrLen(pFilename)));
end; 

Download File Attachment

procedure OpenAttachment(pFileAttachmentEntryNo: Integer)
var
    AttachmentRec: record Attachment;
    ResponseStream: InStream;
    tempfilename: text;
    ErrorAttachment: Label 'No file available.';
begin
    if AttachmentRec.get(pFileAttachmentEntryNo) then
        if AttachmentRec."Attachment File".HasValue then begin
            AttachmentRec.CalcFields("Attachment File");
            AttachmentRec."Attachment File".CreateInStream(ResponseStream);
            tempfilename := CreateGuid() + '.' + AttachmentRec."File Extension";
            DOWNLOADFROMSTREAM(ResponseStream, 'Export', '', 'All Files (*.*)|*.*', tempfilename);
        end
        else
            Error(ErrorAttachment);
end; 

You can download a codeunit containing both functions here.