portal: Use the right mimetype for file transfers
The filetransfer protocol says to use application/vnd.portal.filetransfer, but I used application/vnd.portal.files when I implemented the protocol. Oops.
This commit dds the correct mimetype, but we still support the old one to preserve interoperability with existing flatpaks using GTK 4.6.
Fixes: #5182 (closed)