Allow usage of provided mediatype instead of detected mediatype

[emanueldima]

There are some cases where the provided mediatype is correct but the detected mediatype is more generic and therefore incorrect. For instance https://trep.de.dariah.eu/1.0/dhcrud/21.T11991/0000-001B-4223-1/index which has the mediatype of text/comma-separated-value but is detected as just text/plain. In such cases we should use the provided mediatype instead of the detected mediatype.

See clarin-eric/switchboard-tool-registry#96