The Forge Web Trigger module only accepts requests with a JSON body, as specified in the documentation:

The request is serialized to JSON and passed to the function in the format described below. The function that is invoked is responsible for parsing the JSON data.

The HTTP response is formed from the JSON format described below. If the function result is not compatible with the JSON format, then an error response with status code 500 is sent.

Developers may wish to send & receive additional content types either in the request or response body of the web trigger, for example XML. 

 

This suggestion captures the requirement for other content types to be supported. Please comment on this issue with some information about the content type support you require, if you are affected by this limitation.