In such a scenario, firstly we have to ask for the URL that the client is using and we need to run the GET request using the URL shared by the tenant in Postman to validate what is the content type they are using.
Once we run the GET request in Postman, we need to click the "Headers" section there we will get to know what content type is being used anything apart from 'text/plain' will not work. We need to inform them to use the content type as 'text/plain' only and nothing else.
"MUST set the Content-Type HTTP header to 'text/plain' and nothing else."
Example: In one of the use cases the content type was seen as 'charset' so we informed them to use the content type as 'text/plain' only (below is the screenshot attached for this example/use case).
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This is for the CH team (internal folks)