When I add a new file extension (wdz for example) to the file upload option, in fact it’s allowed but in fact it’ll not work, because WordPress check the mime type of the file and if it’s unknown, WP will block the upload.
I’ve found a “not good” solution with ALLOW_UNFILTERED_UPLOADS and an another plugin that permit to add the mime type, but it will be better if Quform can handle this, or at least display a warning if WP doesn’t know the mime type (get_allowed_mime_types)