Allows for override of SSL replacement.
By default Gravity Forms will attempt to determine if the schema of the URL should be overwritten for SSL. This is not ideal for all situations, particularly domain mapping. Setting $field_ssl to false will prevent the override.
The following would apply to all forms.
add_filter( 'gform_secure_file_download_is_https', 'your_function_name', 10, 3 );
This code should be placed in the functions.php file of your active theme.
This filter is located in class-gf-field-fileupload.php.