Visit https://commons.wikimedia.org/
in the sidebar is "Upload file"

Now we are on
https://commons.wikimedia.org/wiki/Special:UploadWizard
but there is no list of valid upload file types (.jpg, .png, .ogg...).

Look around the page.
See a help link, press it.
OK, now we are on
https://commons.wikimedia.org/wiki/Commons:Help_desk

No answer there.
Must be buried even deeper.

Most other website will mention valid file types even before clicking further.

One must Google to finally find https://commons.wikimedia.org/wiki/Commons:File_types .