portal: Use the right mimetype for file transfers

Matthias Clasen requested to merge fix-filetransfer-mimetype into main

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)

Edited by Matthias Clasen

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