I have the same issue as is explained here, in short:
I am using an iframe to post files to the server AJAX-style, then receiving a JSON response to tell me if the file was uploaded and parsed successfully. This works great is every browser except IE9. Instead, IE9 prompts me to save or open the JSON, and will not pass it back to the iframe.
In the other SO question for this, EricLaw -MSFT suggested to set the content type to "application/json", which I have done, but to no avail.
Does anyone know how to get IE9 to let the iframe have its JSON?