Do you have DataAssist?
The way that I do this is to use the Update Behaviour. Now, to the best of my knowledge you cannot enter a blank value using this, so what I do is place some text in the field I want to update (the file field), say ABC and then save the behaviour so the code is written to the page.
Then go into the code view and simply remove the ABC text from the code so that the string is empty.
You could also have an empty hidden field on the form and bind that to the file field on update, as long as the trigger of the update is your checkbox.
Hope that helps?