I have an ArcGIS 10.5.1 Server with the Image Server extension enabled. Everything is set up the way I want it, and I have two SIDs published to the server for use. However, I've noticed that when I use the services in various maps, including ArcGIS Online as well as Cityworks, the service will throw a 'MIME type ('text/plain') is not executable' error. I'd like to be able to use these services, but I have no idea why this is going on. What do I need to reconfigure to stop this from happening?