FreeCAD is licensed LGPL 2, which means code contributed to it must be compatible with this license. For example, MIT and BSD 3-clause licenses are compatible, but GPL 2 or above are not. The GPL licenses are considered too restrictive; see Licence
In general, CC Attribution-ShareAlike is compatible. It seems that FreeCAD uses CC-BY-SA-3.0, but looking at CC-BY-SA-4.0, it seems to be essentially the same. So either 3.0 or 4.0 should be fine.
But you must allow for commercial use, as LGPL 2 allows it.
In any case, if you want to use a more restrictive license, GPL, or non-commercial, you can always put the files in a separate repository. This is how the Addon Manager
works, it pulls files from other repositories. Since the addons aren't officially part of FreeCAD, they can use any license they choose.