Getting mime type error when calling SOAP from CFC rather than CFM

I am working with a SOAP request in coldfusion. When the code I have is run from a .CFM file, it works. If it is run from a .CFC file, it doesn't work. The code is the same as far as possible (the .CFC needs the cfcomponent and cffunction tags for example).

Why would it work from one and not the other? When I call it from the CFC page I get the error

Unable to determine MIME type of file.

Does anyone have any idea why this might be happening? The SOAP request is identical on both pages. The call with the CFHTTP as well as all the CFHTTPPARAMs are the same as well.