There is a bug that I have noticed for the current version of Mupen64Plus where the file browser cannot find any files. It is not that it doesn’t open, it is that it is literally incapable of seeing any files on your device. It can see folders still, but files just don’t show up. Also, you can’t even use the option for choosing a folder instead of a file, because it still can’t find any files. I think that this is probably some sort of an issue where the android system is only willing to let apps with a specific permission to be able to just see apps at any time.
Mupen 64 devs, please fix this issue, even if you just simply add a setting for using android’s built-in file browser. It is making it to where I cannot get any new roms on the emulator.