Google App Engine: how to send html using send_mail

I have a app with a kind of rest api that I'm using to send emails . However it currently sends only text email so I need to know how to modify it and make it send html . Below is the code :

from __future__ import with_statement

#!/usr/bin/env python
#

import cgi
import os
import logging
import contextlib
from xml.dom import minidom
from xml.dom.minidom import Document
import exceptions
import warnings
import imghdr
from google.appengine.api import images
from google.appengine.api import users
from google.appengine.ext import db
from google.appengine.ext import webapp
from google.appengine.ext.webapp.util import run_wsgi_app
from google.appengine.ext.webapp import template
from google.appengine.api import mail
import wsgiref.handlers

# START Constants
CONTENT_TYPE_HEADER = "Content-Type"
CONTENT_TYPE_TEXT = "text/plain"
XML_CONTENT_TYPE = "application/xml"
XML_ENCODING = "utf-8"
"""
Allows you to specify IP addresses and associated "api_key"s to prevent others from using your app.
Storage and Manipulation methods will check for this "api_key" in the POST/GET params.
Retrieval methods don't use it (however you could enable them to use it, but maybe rewrite so you have a "read" key and a "write" key to prevent others from manipulating your data).

Set "AUTH = False" to disable (allowing anyone use your app and CRUD your data).

To generate a hash/api_key visit https://www.grc.com/passwords.htm
To find your ip visit http://www.whatsmyip.org/
"""
AUTH = {
    '000.000.000.000':'JLQ7P5SnTPq7AJvLnUysJmXSeXTrhgaJ',
}
# END Constants

# START Exception Handling
class Error(StandardError):
    pass
class Forbidden(Error):
    pass

logging.getLogger().setLevel(logging.DEBUG)

@contextlib.contextmanager
def mailExcpHandler(ctx):
    try:
        yield {}
    except (ValueError), exc:
        xml_error_response(ctx, 400 ,'app.invalid_parameters', 'The indicated parameters are not valid: ' + exc.message)
    except (Forbidden), exc:
        xml_error_response(ctx, 403 ,'app.forbidden', 'You don\'t have permission to perform this action: ' + exc.message)
    except (Exception), exc:
        xml_error_response(ctx, 500 ,'system.other', 'An unexpected error in the web service has happened: ' + exc.message)

def xml_error_response(ctx, status, error_id, error_msg):
    ctx.error(status)
    doc = Document()
    errorcard = doc.createElement("error")
    errorcard.setAttribute("id", error_id)
    doc.appendChild(errorcard)
    ptext = doc.createTextNode(error_msg)
    errorcard.appendChild(ptext)
    ctx.response.headers[CONTENT_TYPE_HEADER] = XML_CONTENT_TYPE
    ctx.response.out.write(doc.toxml(XML_ENCODING))
# END Exception Handling

# START Helper Methods
def isAuth(ip = None, key = None):
    if AUTH == False:
        return True
    elif AUTH.has_key(ip) and key == AUTH[ip]:
        return True
    else:
        return False

# END Helper Methods


# START Request Handlers
class Send(webapp.RequestHandler):
    def post(self):
        """
        Sends an email based on POST params. It will queue if resources are unavailable at the time.

        Returns "Success"

        POST Args:
            to: the receipent address
            from: the sender address (must be a registered GAE email)
            subject: email subject
            body: email body content
        """
        with mailExcpHandler(self):
            # check authorised
            if isAuth(self.request.remote_addr,self.request.POST.get('api_key')) == False:
                raise Forbidden("Invalid Credentials")

            # read data from request
            mail_to = str(self.request.POST.get('to'))
            mail_from = str(self.request.POST.get('from'))
            mail_subject = str(self.request.POST.get('subject'))
            mail_body = str(self.request.POST.get('body'))

            mail.send_mail(mail_from, mail_to, mail_subject, mail_body)

            self.response.headers[CONTENT_TYPE_HEADER] = CONTENT_TYPE_TEXT
            self.response.out.write("Success")


# END Request Handlers

# START Application
application = webapp.WSGIApplication([
                                                                             ('/send', Send)
                                                                         ],debug=True)

def main():
    run_wsgi_app(application)

if __name__ == '__main__':
    main()

# END Application

Have a look to the Email message fields of the send_mail function.
Here is the parameter you need:

html
An HTML version of the body content, for recipients that prefer HTML email.

You should add the html input parameter like this:

#Your html body
mail_html_body = '<h1>Hello!</h1>'

# read data from request
mail_to = str(self.request.POST.get('to'))
mail_from = str(self.request.POST.get('from'))
mail_subject = str(self.request.POST.get('subject'))
mail_body = str(self.request.POST.get('body'))

mail.send_mail(mail_from, 
               mail_to,
               mail_subject, 
               mail_body,
               html = mail_html_body ) #your html body