`mime` missing from `bitstream` model

[alanorth]

Currently the bitstream's mimetype is missing from the bitstream model. The client already passes the mimetype in create_bitstream(). If this was available in the model it could be used instead of checking whether the filename string contains ".pdf", for example.

[kshepherd]

Ah yes, there is a HAL link from the bitstream entity for this: https://github.com/DSpace/RestContract/blob/main/bitstreams.md#format

The easiest way to handle this is to embed format in the response like .../bitstreams?embed=format -- and is part of a larger improvement needed, where we expect all sorts of object types in _embedded