Task: Define a MIME type for FCStd files and submit it to IANA

FCStd files have no official MIME type. They either inherit from .zip files and are seen as zip files, or, when the OS allows it, defined by FreeCAD as `application/x-extension-fcstd ` (ex. in https://github.com/FreeCAD/FreeCAD/blob/main/src/XDGData/org.freecad.FreeCAD.xml ).

We should:

- Decide either if we want to keep that "generic" MIME or suggest a more meaningful one (ex. That includes "FreeCAD" in the name). The link below has all the info to help decide
- Register that new name to the IANA. Instructions at https://www.iana.org/assignments/media-types/media-types.xhtml - There is quite a lot of instruction to read.. :) That might be a task for the FPA.

Issue suggested by @pierreporte