Make mimetype check accept json

[luisa-beerboom]

json files (like meeting export files downloaded in the client) currently fail in mediafile.uploads mimetype check.

This should not be the case

[luisa-beerboom]

I tried writing a test for this:
My manually written json content worked as intended.
When I removed everything but the outer brackets it stopped working (probably because python-magic can't determine the type of something so simple, so it goes for text/plain).

Exported meeting from the client also failed even though it really had all the different property types.
My guess is that there may have been something wrong with the files generated by the client, but they looked fine to me. Maybe the client uses the wrong text encoding or smth?