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I have been contemplating the best way to choose an internal file that was uploaded through the filemanager while inside another app and the following is what I came up with (no idea if this has been done before). https://gist.github.com/BronyBorn/1257f75134c57ccce808
This script extends the use of modal.js and filemanager.js to allow apps the ability to have the user pick a domain-relative filepath and then decide what to do with the file's URI once it's been chosen.
If you find something that should/could be improved, please do tell.