Using Telerik Fiddler
to investigate all web requests from the browser, I am unable to find any specific request through the WebResource.axd handler that is sent with no content-type header. You can see the results on my end with this screencast
If this is a behavior that occurs due to a specific ASP.NET or Telerik control configuration, please provide more details about the exact scenario with which the scenario could be recreated locally.
Also, I suggest investigating the requests using fiddler to verify the exact headers of the requests.
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