All-access subscribers can add these fields via the Advanced Applications add-on.

Or you can use this code snippet to add an additional file upload to your application form and adjust the labels/description for more advanced control.
function mdocs_example_matador_application_default_fields( $fields ) {
$field_to_add = [ 'letter', ];
return array_merge( $fields, $field_to_add );
}
add_filter( 'matador_application_fields_defaults', 'mdocs_example_matador_application_default_fields' );
Adjust the labels as needed
add_filter( 'matador_application_fields_structure', function( $fields ) {
$fields['letter']['label'] = 'Upload Additional document';
// optional you can update the lables
$fields['letter']['description'] = 'Attach an additional file, if applicable. Accepted file types are DOC, DOCX, PDF, HTML, and TXT.';
$fields['privacy_policy_opt_in']['options']['1'] = 'By submitting this application, you consent to the collection, use, and disclosure of your personal information in connection with your current application or any future opportunities that match your skill sets.';
return $fields;
} );
If you wish to allow multiple files to be uploaded then you need to use this code
add_filter( 'matador_application_fields_defaults', function( $fields ) {
$field_to_add = [ 'files', ];
return array_merge( $fields, $field_to_add );
} );