typefind: Add _with_extension() variants for typefinding data or a buffer

typefind: Add _with_extension() variants for typefinding data or a buffer

And make use of that in the typefind element to also be able to make use
of the extension in push mode. It previously only did that in pull mode
and this potentially speeds up typefinding and might also prevent false
positives.

https://bugzilla.gnome.org/show_bug.cgi?id=796865