How do I get a string representation of an image before uploading?

I'm trying to intercept an image from an HTML form's input control to convert it into a byte string before processing it on the server side.

How do I intercept the file?

upload_files = self.get_uploads('file')
# Intercept here to do something different than just upload 
blob_info = upload_files[0]

How do I convert it into a byte string that can be converted back to an image later?

I'm using Python and App Engine.

Assume your upload control is in a form named "image" and you are using Werkzeug's FileStorage:

img_stream = self.form.image.data
mimetype = img_stream.content_type
img_str = img_stream.read().encode('base64').replace('\n', '')

data_uri = 'data:%s;%s,%s' % (mimetype, 'base64', img_str)

Your data_uri now contains the string information you need.

Thank you all for your helpful comments!