rest: change default mime type for files download

[mdonadoni]

Changes the default MIME type used when downloading files to application/octet-stream.
Previously, multipart/form-data was used but the response was not correctly encoded as such.

Closes reanahub/reana-server#418

How to reproduce the issue:

1. Launch reana-demo-helloworld
2. Preview the file data/names.txt
3. Check the logs of reana-server

Expected result: the warning is not present anymore in reana-server's logs

[VMois]

Warning disappeared.

minor: Looks like the return "Content-type" is always "application/octet-stream" for text, python, and JSON in web UI. Images and HTML files are fine. Not sure whether the client receives the same content type as web UI. It should use the same endpoint for downloading files.

Maybe, we should remove "application/json" from the OpenAPI spec if the JSON content-type is never returned.

[mdonadoni]

minor: Looks like the return "Content-type" is always "application/octet-stream" for text, python, and JSON in web UI. Images and HTML files are fine. Not sure whether the client receives the same content type as web UI. It should use the same endpoint for downloading files.

Yes, even before these changes the content type was always multipart/form-data. However, in these cases (Python/JSON/text files) it might be better to use text/plain. Should we open an issue about this?

Maybe, we should remove "application/json" from the OpenAPI spec if the JSON content-type is never returned.

The application/json type is used when returning errors, for example:

reana-workflow-controller/reana_workflow_controller/rest/workflows_workspace.py

Lines 273 to 278 in 1aaeacf

	except KeyError:
	return jsonify({"message": "Malformed request."}), 400
	except NotFound:
	return jsonify({"message": "{0} does not exist.".format(file_name)}), 404
	except Exception as e:
	return jsonify({"message": str(e)}), 500

[VMois]

However, in these cases (Python/JSON/text files) it might be better to use text/plain. Should we open an issue about this?

Let's open an issue not to lose track of it. I am not sure how important it is, but it doesn't break stuff so let's have an issue and deal with it later.

[VMois]

PR solves the issue. Nice work!

If you can create a minor issue to investigate/fix encoding type problems later (for text, py, and JSON files), that would be great.