2282307 attachment endpoint

Implementation of Attachment endpoint. 

What is not yet implemented/blockers: 
- For get request the value of "Content-Type" header is not correct in the response headers, it should be the mime type of the attachment.
- On put request the file should be attached to the entity as a new value for field_test_file.
When we create a generic `EntityUuidConverter` this should become `{entity_uuid:file}`. This will indicate that the converter is of the primary type "entity_uuid" and will give the converter information about the entity type as well ("file" in this case).