Avoid adding on duplicate extensions.

As we often use filenames for datastream labels, and then use the labels
to generate file names for download, we were ending up with file
extensions being doubled up.

includes/datastream.inc

@@ -53,8 +53,16 @@ function islandora_view_datastream(AbstractDatastream $datastream, $download = F
   if ($download) {
     // Browsers will not append all extensions.
     $mime_detect = new MimeDetect();
-    $extension = $mime_detect->getExtension($datastream->mimetype);
-    $filename = $datastream->label . '.' . $extension;
+    $extension = '.' . $mime_detect->getExtension($datastream->mimetype);
+
+    // Prevent adding on a duplicate extension.
+    $extension_length = strlen($extension);
+    $duplicate_extension_position = strripos($datastream->label, $extension, -$extension_length);
+    $filename = $datastream->label;
+    if ($duplicate_extension_position === FALSE) {
+      $filename .= $extension;
+    }
+
     header("Content-Disposition: attachment; filename=\"$filename\"");
   }