the media types in the Accept header is something that your browser send in the request to the server. It tells the webserver that the browser will only accept certain kind of files. This is not directly related to the media type system list that MWG uses. This list is usually used for the actual response coming from the server when MWG will try to detect the actual media type of the file.
Normally MWG does nothing with the Accept header that the browser send, it's very unlikely that this is the reason why a webpage is not working. This would only be the case if you wrote a rule yourself that performs block actions especially for this header.
The reason why something is not working is more likely based on the response from the webserver, which is processed by MWG.
Without further information it's not possible to detect the issue or say what's causing it, but I'm very sure that it's not the media type list.