diff -r 000000000000 -r 6474c204b198 docshell/test/unit/test_bug414201_jfif.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/docshell/test/unit/test_bug414201_jfif.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,27 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Test for bug 414201
+ */
+
+function run_test()
+{
+    var ms = Components.classes["@mozilla.org/mime;1"].getService(Components.interfaces.nsIMIMEService);
+
+    /* Test a few common image types to make sure that they get the right extension */
+    var types = {
+        "image/jpeg": ["jpg", "jpeg"], /* accept either */
+        "image/gif": ["gif"],
+        "image/png": ["png"]
+    };
+
+    /* Check whether the primary extension is what we'd expect */
+    for (var mimetype in types) {
+        var exts = types[mimetype];
+        var primary = ms.getFromTypeAndExtension(mimetype, null).primaryExtension.toLowerCase();
+
+        do_check_true (exts.indexOf(primary) != -1);
+    }
+}