It made sense (at the time) to be able to disable the upload fields from the export. You're not always interested in a long url, so you could (and still can) disable this via a setting on the General Settings page.
Of course nowadays you can just disable specific fields on the form settings, but it's still a nice feature to have.
Makes it possible to disable/enable a field. Just return a boolean, or use the wordpress
// all upload field are excluded from the export. add_filter('gfexcel_field_fileuploads_enabled', '__return_false');