I asked that already, but here is how another dev answered they found the option to activate it for the Itch.io app.
once you upload a file, when editing the post, in the files section, under the name and size of said file, there's a list where you have to state the type of file it is (executable, video, etc.) and at the side of it, there are some easy to ignore small checkboxes, from left to right they have the windows, linux, apple and android logo.
These checkboxes, that do nothing in the web version, are the ones that the app check to see if a file is downloadable in your system.