Only allow specific file extensions in angular-file-upload module to be clickable

Question

I'm using this module, https://github.com/nervgh/angular-file-upload

I am using their simple example, http://nervgh.github.io/pages/angular-file-upload/examples/simple/

When you click "Choose file" button, it will open a window which allows us to pick a file that we want to upload. What I would like to happen is to only allow specific file extensions to be clicked. Is there a non-documented configuration which will do that?

For example, in the config, we only allowed .txt extensions, when I click "Choose file" button, it will open up a window and show all the files. However, files that doesn't have the extension .txt will be disabled.

I'm not doing this for security purposes. I am just following the design/instructions provided to me. :) I can always use mitm tools to intercept and allow other extensions to be uploaded.