Added 'islandora_' to hook name.

includes/datastream.inc

@@ -65,7 +65,7 @@ function islandora_view_datastream(AbstractDatastream $datastream, $download = F
     }
 
     // Allow other modules to modify or replace the filename.
-    drupal_alter('datastream_filename', $filename, $datastream);
+    drupal_alter('islandora_datastream_filename', $filename, $datastream);
 
     header("Content-Disposition: attachment; filename=\"$filename\"");
   }

islandora.api.php

@@ -955,7 +955,7 @@ function callback_islandora_breadcrumbs_backends(AbstractObject $object) {
  * @param AbstractDatastream $datastream
  *   The datastream object being downloaded.
  */
-function hook_datastream_filename_alter(&$filename, AbstractDatastream $datastream) {
+function hook_islandora_datastream_filename_alter(&$filename, AbstractDatastream $datastream) {
 
   // Example taken from islandora_datastream_filenamer.
   $pattern = variable_get('islandora_ds_download_filename_pattern', FALSE);