Preview: Fix initial audio preview

This fixes the initial preview for audio files (link received > file downloaded > preview generated). Previously, Gajim tried to guess the MIME type of the received file before it was actually written to disk (because writing is async). This lead to the case where MIME type could not be guessed, because the file has not been written yet.

With this patch, Gajim updates the preview only after writing the file to disk. If a thumb image has been displayed already, Gajim does not update the preview.