Update app.py

Thanks for your code.  I got an error running python app.py -- ImportError: No module named flask.ext.wtf.FileField
I got it to run after reading the following: 
https://flask-wtf.readthedocs.org/en/v0.9.5/form.html
http://stackoverflow.com/questions/18297041/im-not-able-to-import-flask-wtf-textfield-and-booleanfield
Add UTF-8 encoding to example.html 

from http://docs.aws.amazon.com/AmazonS3/latest/dev/HTTPPOSTForms.html 

HTML Form Encoding

The form and policy must be UTF-8 encoded. You can apply UTF-8 encoding to the form by specifying it in the HTML heading or as a request header.

Note

The HTML form declaration does not accept query string authentication parameters.

The following is an example of UTF-8 encoding in the HTML heading:

<html>
  <head>
    ...
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
    ...
  </head>
  <body>
Merge pull request #2 from xpat/master

Update app.py