I poked around and I found what it does.
After adding a App using "Manage App", and you can add a File Opener, and specify a file to be opened with App.
For instance, if you want to open a text file with different editor you can specify your text editor as an App, then Add a "File Opener" widget using the text editor you specified.
@alex Thanks for the reply. I really appreciate it. Instead of rebuilding the wheel, how about put them all into a folder and let user to use google drive, onedrive, dropbox whatever they are using. Freeter can just specify a directory for storing the data, which can be located anywhere. This how some other apps approach with sync, for instance, https://joplinapp.org/help/apps/sync/dropbox
