edit form: default value file field
According to the FAPI documentation, #default_value isn't a valid property for a file element which is probably why adding it to the array has no effect.

You might have better luck using a managed file element instead; although the documentation also states that a #default_value is not valid for that type either, I've used it successfully for ages now. The default value needs to be a file id (fid) which references a file in the file_managed table.