Using Edit Schema to create/edit a Form. For the field in question, I check the "This field can hold a list of values" checkbox and set the "This field will always have values in the following namespace:" to File.

For the Form Input section, I set the input type to "text with auto complete" and enter "values from namespace=File, list" into the Enter parameter names textbox.

After generating pages, the form shows the list parameter as list= instead of list. I can manually change this, so it's an easy workaround, though tedious when needing to make changes often.