Per T208494.

MuseScore is free and open-source software. Although there is no existing MediaWiki extension that could generate thumbnails for it, the software itself can export files to PNG, SVG, OGG and PDF.

The .mscz format is compressed, and the .mscx format is not (this is the only difference). Compression reduces file size by about 1.5 orders of magnitude, which may be significant. If there are unresolvable issues with allowing compressed files to be uploaded, then only .mscx would be sufficient; otherwise, if there are no such issues, then only .mscz should be supported.

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.

It would be possible, but possible undesirable, to use the Score extension MediaWiki-extensions-Score to generate thumbnails. This currently requires a two-step file conversion through MusicXML; the three formats are not totally compatible with each other. There may also be undesirable visual issues.

MuseScore 3.0 was released on 24 December 2018. 3.0.1 was released on 15 January 2019. There are unresolved issues with the file format, such as the fact that there is no formal specification, and that breaking changes (introduced by versions 2.0 and 3.0) can cause older files to become unusable in newer versions of the software. This is usually not an issue for end users, but it would make it difficult to implement a MuseScore extension without using the MuseScore source code.