Current version of J2Store is 3.3.10 and if you intent to sell file by it, maybe this article written for you! In Backend of J2Store, you need to select downloadable item first and in [file] tab, there is a button with name "set product file"
When you want to select uploaded file with J2Store browser (honestly I don't like UI!) there is a bug and that's about vertical scroll bar. If you have a lot of files here, you can not scroll down the list to see end of it.
I think this is a CSS issue and will be fixed in next versions but for now you can follow these steps.
1 If you are using ISIS template for backend you need to open below file :
Or make it if there isn't a custom.css in the css folder.
2 add below codes to custom.css save the file and refresh backend completely (CTRL+F5) :
Now the problem should be fixed!