Add Content-Disposition Header to file download

[temparus]

We should add the Content-Disposition header for the media endpoint. This allows us to suggest a filename to the browser for saving. If a user wants to save a file, it just takes the file id as the file name (without any file ending).

I would suggest that we add the following header:

Content-Disposition: inline; filename="myfile.txt"

The media endpoint [1] is handled completely by Python Eve. Changing this part of the Eve Framework would be necessary.

[1] https://github.com/pyeve/eve/blob/b5ca5ce8d91cb2081eaf5c14ff2e6676c12c73e6/eve/endpoints.py#L182