This causes weird UX:
- In firefox for example, after each pause in typing inside the filter field the search-as-you-type feature fires up and focus switches to the browser search box. One needs to click inside the filter field again and again in order to continue filtering the list of files.
- Hitting the backspace takes you to the previous page. This forces people to re-edit and re-click the link that launches the "Select a file" dialog. A lot more clicks for the user to get back to they were.