Get list / filepaths of all files in a folder and subfolders

Just Evan's icon

Hello!

How do I get a list of all files / filepaths to fill the menu?

I think it's pretty easy, but I'm missing something.

Source Audio's icon

read umenu help and reference.
keywords are prefix, autopopulate and filetype

Roman Thilenius's icon

you can also list files of type "fold" - which are folders - and then list their content somewhere else.