GTK 3 uses application/vnd.portal.files, should be ….filetransfer
GTK 3.24.37 added support for the file transfer portal backported from GTK 4, but it seems to be missing the equivalent of !5033 (merged): the code mentions
application/vnd.portal.files but not the recommended
application/vnd.portal.filetransfer from https://docs.flatpak.org/en/latest/portal-api-reference.html#gdbus-org.freedesktop.portal.FileTransfer. Presumably it should accept both and treat them as equivalent?