When drag-and-dropping a file from Nautilus to for example Firefox, this
does not work if the file is from a GVFS mounted source. The retrieved
URI with `g_file_get_uri()` still contains the protocol.

Instead, we can use `g_filename_to_uri()`, which resolves to the local
`file://...` URI instead.

A similar fix was applied to GTK4 on the sending side in commit
ea056d26.