Exclude non-video media formats in FFmpeg backend

Qt Multimedia forwards files to FFmpeg for rendering, and we don't put restrictions on what kinds of files we support. This means that we support rendering text and images, but the result may not be as expected. For example, playing a single picture will just flicker for a short time on the screen.

We should likely define a set of formats that we support, and exclude unsupported formats such as text or pictures.