uploads: fix double paste bug.

Fix a bug where, when pasting an URL directly into the URL field, the
URL would be pasted twice, which would cause the upload to fail because
the URL was invalid.

app/components/file_upload_component/file_upload_component.js

@@ -81,6 +81,8 @@ export default class FileUploadComponent {
     if (/^https?:\/\//.test(url)) {
       this.$component.find("input[type='submit']:not([disabled])").click();
     }
+
+    e.preventDefault();
   }
 
   // Called after the upload is submitted via AJAX. Polls the upload until it