Fix non-unique input name for a non-cloneable file in a cloneable group

Author: rilwis

inc/fields/file.php

@@ -358,7 +358,7 @@ public static function normalize( $field ) {
 		);
 
 		$field['multiple']   = true;
-		$field['input_name'] = "_file_{$field['id']}";
+		$field['input_name'] = '_file_' . uniqid();
 		$field['index_name'] = "_index_{$field['id']}";
 
 		return $field;