> It would make more sense if the file picker was something the operating system ran when a program wants to edit a file, and what came back to the program after you selected was the accessor for that file (with read and/or write verbs).
You are describing the concept of XDG Desktop Portals [1] on Linux.
Indeed, that strikes me as a fine example of capability inspired design. The mechanism used is passing file descriptors, and for some reason file descriptors is the most "capability based" part of the Linux kernel.
1. https://wiki.archlinux.org/title/XDG_Desktop_Portal