CURA-3975
This is a temporary fix to make materials work with 2.7 version upgrade
because of the setting_version change from 1 to 2. This MUST be fixed
after we have decided on how to determine the versions of an
XMLMaterialProfile.
Because of this wrong MIME type, the material profile would get saved using the preferred extension of the instance container, .inst.cfg. This would then get interpreted as having the MIME type of instance containers, and then it would fail to load because it is not an INI file.
Now it stores it as .xml.fdm_material and so the MIME type database interprets it as an XML file.
Contributes to issue CURA-3937.
