Drupal 7, form api - file field default_value
No, there's no way to do this. The restriction is with HTML, not Drupal. A file input simply doesn't have the capacity for a default value to be set; allowing such behaviour would introduce a security hole (imagine you pre-filled it with /~/.ssh/id_rsa, auto-submitted the form, and the user didn't have permissions set up correctly). See this post.

If you check the FAPI guide you'll see there's no #default_value available for a file field type.

You'll need to switch to a managed_file element if you need a default value for the field.