Verify all files in the request body are accessible by the renderer process.

content/test/data/form_that_posts_to_echoall.html

Index: content/test/data/form_that_posts_to_echoall.html
diff --git a/content/test/data/form_that_posts_to_echoall.html b/content/test/data/form_that_posts_to_echoall.html
index ddd726e05ae021ab7e3075d5eac882e47fe1757a..161f84616fc5dc18577588c862003402dbe5119a 100644
--- a/content/test/data/form_that_posts_to_echoall.html
+++ b/content/test/data/form_that_posts_to_echoall.html
@@ -6,5 +6,9 @@
       <input type="text" name="text" value="value">
       <input type="submit">
     </form>
+    <form id="file-form" method="POST" action="/echoall" enctype="multipart/form-data">
+      <input type="file" id="file" name="file">
+      <input type="submit">
+    </form>
   </body>
 </html>