Maybe those files were uploaded before this "autorename" was introduced. Are you sure this autorename isn't happening now at upload time?

Ciencia_Al_Poder: Here is a link to a .ogg file that was uploaded yesterday:

https://commons.wikimedia.org/wiki/File:Pa-%E0%A8%89%E0%A8%82%E0%A8%97%E0%A8%B2.ogg

Its extension did not change at the time of uploading.

File extensions are normalized when uploading with UploadWizard, but not when using other tools. Some of the rules are different than what you propose (e.g. we normalize .ogv to .ogg): https://github.com/wikimedia/mediawiki/blob/f7d75435afe6589ee4b05f02259def121d633954/resources/src/mediawiki/mediawiki.Title.js#L767. That was implemented per T36703. There are old tasks about doing this everywhere, e.g. T34660, T31284. It's not trivial, if only because we already have probably millions of files with names that would become incorrect if this was done. This would likely be a duplicate of one of those tasks.

But I'd like to go back to the original issue – "File format changes automatically while moving Audio Files". I am pretty sure that MediaWiki does not do this. This has to be caused by some Commons-specific customization; I don't see immediately what would do this. I guess I'll need to get filemover rights and see for myself.

Fair enough.

I didn't know about this change on renaming files, but I'm unable to reproduce it. on my local MediaWiki install downloaded from git. I've downloaded that file, uploaded it, and renamed, and it still has the .ogg extension.

Automatically changing the file extension when a file is moved, without issuing a warning to the user, is a very bad decision IMHO.

I'm pretty sure this happens in the confusingly-named https://commons.wikimedia.org/wiki/MediaWiki:Gadget-AjaxQuickDelete.js, in the cleanFileName method. It replaces the file's extension with mimeFileExtension, calculated earlier from the MIME type of the file.

This task would be a duplicate of T31284 as originally filed and I think the real issue is elsewhere, so I changed the task title to be about that.

https://commons.wikimedia.org/wiki/MediaWiki:Gadget-AjaxQuickDelete.js, search for this.mimeFileExtension = 'oga';