ASP.NET Ajax Toolkit Slider Pen Displays Broken Image

Question

Using the ASP.NET 4.5 Web Forms project, which has been working for many years, and a new problem has arisen: ASP.NET Ajax Control Toolkit Slider stops rendering the image (its broken image symbol, where the “descriptor” is usually displayed).

This only happens in IE11. It works fine in Chrome or Firefox (below):

We are currently reviewing the latest code changes (we are conducting an annual security review at OWASP Top 10) to see what could break it, but any advice has been appreciated.

Edit: we recently added entries to our web.config customHeaders section for https://scotthelme.co.uk/hardening-your-http-response-headers

When we set X-Content-Type-Options = nosniff to web.config as shown below:

</system.webServer> ... <httpProtocol> <customHeaders> <add name="X-Content-Type-Options" value="nosniff" /> </customHeaders> </httpProtocol> </system.webServer> 

the slider is broken into E11. When the above is removed, the slider works in IE11.

I do not quite understand why, but this seems to be our culprit.