Less unnecessary datastream versions on replacement.

includes/datastream.version.inc

@@ -344,7 +344,9 @@ function islandora_datastream_version_replace_form_submit($form, &$form_state) {
   $file = file_load($form_state['values']['file']);
   try {
     $ds = $object[$form_state['dsid']];
+    if ($ds->mimetype != $file->filemime) {
       $ds->mimetype = $file->filemime;
+    }
     $path = drupal_realpath($file->uri);
     $ds->setContentFromFile($path);
     file_delete($file);