[Bug 767268] Add missing filename type annotations

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=767268

--- Comment #9 from Christoph Reiter (lazka) <reiter.christoph at gmail.com> ---
I got one response and no objections. See
https://mail.gnome.org/archives/gir-devel-list/2016-June/thread.html#00000

So I'd like to propose the original patch for inclusion.

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