Index: wp-admin/includes/media.php
===================================================================
--- wp-admin/includes/media.php	(revision 21255)
+++ wp-admin/includes/media.php	(working copy)
@@ -1328,7 +1328,7 @@
 <?php
 // Check quota for this blog if multisite
 if ( is_multisite() && !is_upload_space_available() ) {
-	echo '<p>' . sprintf( __( 'Sorry, you have filled your storage quota (%s MB).' ), get_space_allowed() ) . '</p>';
+	do_action( 'upload-ui-over-quota' );
 	return;
 }
 
@@ -2065,6 +2065,11 @@
 }
 add_action('post-plupload-upload-ui', 'media_upload_flash_bypass');
 
+function multisite_over_quota_message() {
+	echo '<p>' . sprintf( __( 'Sorry, you have used all of your storage quota of %s MB.' ), get_space_allowed() ) . '</p>';
+}
+add_action( 'upload-ui-over-quota', 'multisite_over_quota_message' );
+
 /**
  * {@internal Missing Short Description}}
  *