Taken from Muhammad Bilal Awan's post below but pasted here for my own benefit;
Open IIS Manager
Display properties for the IIS Server
Click MIME Types and then add the JSON extension:
File name extension: .json
MIME type: application/json
Go back to the properties for IIS Server
Click on Handler Mappings
Add a script map
Request path: *.json
Executable: C:\WINDOWS\system32\inetsrv\asp.dll
Name: JSON
And the same applies for geojson of course.
http://www.uipress.com/add-json-handler-support-in-iis-7/#.UPax0mKalYh