File upload hangs when uploading unsupported files (e.g. wmv)

[micheleidesmith]

See https://www.evernote.com/shard/s6/sh/e51b29e5-5edf-49c4-87ed-b12991441ff8/e475fbd19a753485fd223c8cbe9a1b1f

We should review what files we allow users to upload.

[timdegroote]

Closing as https://github.com/CUL-DigitalServices/avocet-ui/pull/354 has been merged.