The ToolbarMiddle accesses request.POST, which causes the request data and/or files to be parsed, and prevents any views in the CMS from being able to modify the upload handlers for the request as the request body has already been parsed, and and request._files has already been populated.
Middleware that accesses request.POST should generally be considered a bug - Eg see Jacob K-M's comment on this ticket: https://code.djangoproject.com/ticket/613
I'm not quite sure of the right way to fix this. It's probably fine for the middleware to access .POST if either:
Fixed #897 by only accessing request.POST in the middleware when we'r…
…e pretty sure the POST is for the toolbar (login)