Since march, there is now an official MIME type for STL files. See https://www.iana.org/assignments/media-types/model/stl for the new MIME type specification.
Fixes #4141.
* Originally taken from my own packaging of Cura on launchpad: ppa:thopiekar/cura - https://launchpad.net/~thopiekar/+archive/ubuntu/cura
* It adds mime-types, which are supported by Cura, to the mime-database, so filemanagers can recognize these files.
* This sharemimeinfo includes currently 3MF, STL and OBJ.