Post-GTK4 migration: support more image formats

[v1993]

We have Gtk.FileFilter.add_pixbuf_formats() that allows to easily allow opening a wide arrange of image formats.

An issue remains with selecting an appropriate file extension when uploading, with a few solutions available:

• Parse mime database ourselves. There's a function in GLib to get list of active mime directories. The correct solution, but a lot of code for such a small problem. Might as well look into contributing to GLib at this point...
• Use extension of originally selected file, which should produce good results in most cases.
• Support PNG/JPG/TIFF and convert everything else to one of those formats during uploading - Pixbuf methods are available to do this easily.

The last two options can be exposed to end user to let them decide themselves.