switch to posttestserver.com for IE compatibility

website/src/examples/multipart/app.html

@@ -2,7 +2,7 @@
 <link rel="stylesheet" href="/uppy/uppy.min.css">
 
 <div class="UppyForm">
-  <form action="//api2.transloadit.com">
+  <form action="http://posttestserver.com/post.php?dir=uppy&status_code=202" enctype="multipart/form-data" method="post">
     <h5>Uppy was not loaded — slow connection, unsupported browser, werid JS error on a page — but upload still works, because HTML is cool like that</h5>
     <input type="file" name="files[]" multiple="">
     <button type="submit">Dumb Fallback Form Upload</button>