MP3 upload using USB to Librem 5 local storage?

How would this work? Using a special USB device?

I think the idea would be to use a USB cable from another device, like a laptop. This issue was opened to address use cases like the one you are asking about, I think.

I personally solve this issue with syncthing: i have a directory on my notebook which mirrors the mobiles content. if i want something on my mobile i just copy it there (on my notebook) and syncthink will sync it to the mobile.

All syncing is done via network (limited to my home wifi in my case).

Syncthing will be available for Librem5 i assume.

Be aware that MTP is broken by design, i would not recommend that to be used on Librem5 (or any other phone).