Depending on the Android OS, you might be able to have a Markdown URL like
file:///sdcard/path/to/file. I say might because it's security hole and I'm betting most recent Android OSes have blocked it.
One alternative is to use a file widget and have a default value set to say
../../forms/My Form Title-media/my-media-file.txt, then, assuming that media file is included in the blank form, when that widget loads and you hit the icon, it'll load
my-media-file.txt with whatever app on the device that loads txt files.
Try the above and if it doesn't feel like a good solution, perhaps you can tell us more about the high-level problem you are trying to solve.