By adding a header in the head section of your HTML, you can enable IE7 compatibility mode.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
<meta http-equiv="X-UA-Compatible" content="IE=7" >
It’s also possible to add this header in IIS (assuming you are using IIS, in other webservers probably a similar approach is available) to enable it site wide, if you don’t have the freedom to add the header in the HTML of existing websites.
Note: Enabling compatibility mode can have consequences for other parts of your website as well, be sure to properly test everything before applying this change
This gets the plugin to work properly under IE9 it’s probably not the best solution but it’s a workaround that fixed it for me in this scenario.
Sidenote: Make sure that the response of the upload is returned with the content type text/html IE doesn’t know what to do with the content type application/json.