How to change "Description" in File field to drop down instead of text field?

Question

What is the best way to modify the File field so the "Description" field is a select box instead of a text field. I.e. I want to go from:

To this:

I tried a quick experiment, editing the File modules file.field.inc file, which resulted in the screenshots above, but if I can avoid forking the File module and instead do it some other way it would be great.

Answer 1

You can use hook_field_widget_form_alter() to add your own #process callback to the element.

In the callback you can just change the description field from a textfield to a select.

Answer 2

Turns out the easiest solution is to use the Field Collection module, which allows me to do just what I want.