Set default app for file type NOT recognised I'd like to associate an app with a file type that android does not recognize.
I am using a samsung tab 3 with android 4.2.2.
In my case I have a .tex file and 2 apps that can read it, but when I click on the file from "My files" app, it says 
"Open file
Unable to find application to perform this action"

How can I tell it it to associate the readers with this file format?
When you do an action like viewing a file, Android shows the chooser to let you choose from all the apps that say they can understand that file or URL. It's up to the app author to tell Android what files the app can understand. If an app doesn't show up in that list when you think it should, it's a bug in the app, and you should contact the app developer to ask them to make that change.

It's possible to use a third-party app to act as an intermediary, to let you send any action on any file or URL to any app of your choice, but the result will almost always be that the 'target' app crashes, or just opens to the main screen without opening the file you want, because it hasn't been programmed to understand that intent.