Upload MP3 directly to song on Musescore Website, not from Musescore App

As of right now, I can only find two ways to upload a song with a custom audio: I either upload it online from the app using Save Online, or I can choose some audio from Youtube. I really wish there was just an option to add your own MP3/other music file directly using the website, especially since I'm unable to give my older compositions unique soundfonts on the website.

For reference:
Here is a song where I uploaded by putting the file in the upload folder: https://musescore.com/greg_frin/a-queen-so-machine
Here is a song where I uploaded using the Save Online option (notice that there's the correct soundfont for the pipa, which doesn't exist in Musescore's audio library): https://musescore.com/greg_frin/the-oriental-of-the-desert

Now say I want to add the pipa to my first song. I can't use Save Online because it'll keep making a new page (unless I'm doing it wrong, in which case please help!), and making a whole youtube video would be kind of tedeous (plus for me personally, I frequently update my music :P). If there was an option in "Manage audio sources" that would allow us to upload our own MP3, this would be a whole lot easier to work with! Please help!