How to add a menu behind "Upload Files..."?

I see “CommonMenus.FILE_AUTOSAVE”, “CommonMenus.FILE_AUTOSAVE” and so on, but i just can not find any code like “CommonMenus.Upload” and “CommonMenus.Download”. So how can i add a menu behind “Upload Files…”?

[original thread by PeimingMo]

They are coming from @theia/filesystem extension. Use search in workspace to find a command by a label in the code.