Per T208494.

MusicXML is a platform-agnostic exchange format. This also means that there is pretty much no software which prefers it over the software's native format. Regardless, because of the large number of proprietary programs which can export to MusicXML, enabling Commons uploads would still be useful and was supported in the RfC.

There is no requirement to enable thumbnails or playback before it becomes possible to upload files to Commons; there was (I think) no expectation in the RfC that this would occur immediately due to the Community Wishlist Survey proposal for such not reaching the top 10.

As with MuseScore, .mxl is the compressed format and .musicxml is the uncompressed format; compression reduces file size by about 1.5 orders of magnitude. If there are unresolvable issues with uploading compressed files then .musicxml would be sufficient; however, if there are no such unresolvable issues then only .mxl needs to be supported.

If rendering for MusicXML is enabled, it would have to be decided what software to use to render it. The Score extension (LilyPond), MuseScore and Verovio (MEI) are all possible contenders, although if there is no developer interest in creating extensions for MuseScore or Verovio then the obvious choice would be to import MusicXML files into LilyPond server-side. For what it's worth, Verovio has a jsfiddle demo (1.4 KB of code) as well as its full MEI/MusicXML viewer.