Add file type suffix to file name when saving to gallery

[ichthyosaurus]

Currently, comic strips are saved without ".jpg", ".png", etc. suffixes. While file manager apps and the native gallery app don't care, the files do not show up in the Android gallery.

This patch uses Qt's built-in mime type features to add the correct suffix when saving to the gallery.

[Maledictus]

Looks good. Thank you.

[ichthyosaurus]

That was quick, thanks :).