I'm afraid this is a hard problem. As the file_upload uses Media Library to upload files, all it does is calling the popup, passing the required parameters for the uploader (mime types, file size, etc.) and parsing the returned data. I've just checked the code and couldn't find anything related to the current field, because the uploader doesn't need that. But can you try print_r everything global ($_POST, $_GET, etc.) when filtering? Probably I'm missing something.