- * - rename: (optional) If set, a unique destination URI is generated. If not
- * set, the destination URI will be overwritten if it exists.
+ * - file_exists: (optional) Replace behavior when the destination file already
+ * exists:
+ * - 'replace' - (default) Replace the existing file.
+ * - 'rename' - Append _{incrementing number} until the filename is
+ * unique.
+ * - 'use existing' - Do nothing and return FALSE.