It should be possible to upload MusicXML (.mxl) and/or MuseScore (.mscz) files to Wikimedia Commons. This would make it easier to share public-domain sheet music and would enable users to more easily edit each other's scores. (Currently, scores are either uploaded to Commons as images or used directly in pages through the Score extension, with benefits and disadvantages for each approach.)
Files would be rendered either by LilyPond through the Score extension (T201637) or by MuseScore's GPL application through a new extension. In addition, it may also be desirable to allow the score files to be rendered as audio using the normal file syntax.
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[x] Check if no one has beat you to it (Score extension exists, no support for other file types)
[x] make sure the file type is one of the filetypes allowed for uploading (none of them are)
[] Determine whether thumbnail generation is necessary before allowing uploads
[x] Do you need the file format to be supported on WMF websites? (yes)
Support for uploads
[] Add support for the MIME types
[] Add the file types to $wgFileExtensions
Thumbnailing and embedding
[] Determine if you need support in MediaWiki core
[] Create MediaHandler subclasses for filetypes
[] Register your MediaHandler
WMF sites checklist
[x] Free and open
[] The formats should not allow execution of code (not completely sure)
[] Security review of new extensions
[] Thumbnail format should be commonly supported (PNG definitely possible for LilyPond/ABC/MuseScore/MusicXML, PDF possible for all except Braille)
[] Thumbnail (esp. when interactive) should be usable and accessible to screen reader users(?)
[] Interactive content should not reflow the page
[] Interactive content should perform (JS needs to be added only to the pages that require it, etc.)
[] If files exceed $wgMaxImageArea, special support is needed
[] thumbor engine
[x] Community RFC