Take a look at some JS solutions for AJAX upload - specifically, Plupload can be hooked up to work with the App Engine blobstore, giving you multiupload support, AJAX upload, and options for upload widgets/progress bars/etc.
In fact, @NickJohnson has a full blog post guiding you through the steps.
The gist of it is:
1) Download and install Plupload
2) Create a handler that returns a generated upload URL. Something like this:
from google.appengine.ext import webapp
from google.appengine.api import blobstore
class BlobstoreURLResponder(webapp.RequestHandler):
""" Mapped to the URL /get_upload_url """
def get(self):
self.response.headers['Content-Type'] = 'text/plain'
self.request.out.write(blobstore.create_upload_url('/blobstore/passthrough'))
3) Hook up Plupload to get a blob upload URL before uploading a file
uploader.bind('UploadFile', function(up, file) {
$.ajax({
url: '/get_upload_url',
async: false,
success: function(data) {
up.settings.url = data;
},
});
For more detailed instructions, take a look at that blog post. Nick has an awesome walkthrough that definitely helped me get set up with Plupload + Blobstore.