libreoffice can hang in an infinite(?) loop

[blizzz]

Observed it with an ogg file. LibreOffice would try to treat it with the Writer import filter and just run and run and run as waste CPU cycles but don't come to an end (I waited about half an hour). Audio- and Video mimetype now are black listed, howerver I would not trust LibreOffice be working properly with other unexpected formats or perhaps corrupted files. Since the libreoffice would run, the cron job would run, too, and not finish, thus block everything.

To avoid this, we shoud switch from using exec to proc_open and friends which allow more control, for instance cancelling the process after some time. Cf. https://github.com/nextcloud/pdf_converter/blob/master/lib/BackgroundJobs/Convert.php#L87-L95