Request bodies can be of multiple IANA media types, with application/json being the most common media type used across modern web APIs. However, form, PDF, XML, and other media types are common place and should be considered as part of governance of media types for request bodies.
Depending on the legacy of your operations I recommend using application/json as the default and require exceptions to be added for any others, then proposing new policy rules to help gov ern the payload of POST, PUT, and PATCH API requests. This allows you to start with the basics and evolve your governance of the media types for request bodies over time, using an evidence based approach.