+'ImgAuthModifyHeaders': Executed just before a file is streamed to a user via
+img_auth.php, allowing headers to be modified beforehand.
+$title: LinkTarget object
+&$headers: HTTP headers ( name => value, names are case insensitive ).
+ Two headers get special handling: If-Modified-Since (value must be
+ a valid HTTP date) and Range (must be of the form "bytes=(\d*-\d*)")
+ will be honored when streaming the file.
+