Notice how your browser always changes somethings when you get to another page in the same site, for example, when you go through the forums clicking on topics and the URL changes? That's a path.
But since opening files work a little different, you'll have to present a "path" for the browser, so that it can attempt to load the file.
-Example for a pc file: file:///C:/Users/[User]/Desktop/blah.file
-Example for a android file: file:///sdcard/download/blah.file
It might be the same way on iOS, always start with "file:///..." then you input the path, which in a android phone case, starts with "sdcard" and so forth. You'll have to check your phone's storage name yourself though.