This is a missing capability in our Batch content support. The details are here in the OData spec.

For media type application/json or one of its subtypes, optionally with format parameters, the value of body is JSON.

For media types of top-level type text, for example text/plain, the value of body is a string containing the value of the request body.

For all other media types the value of body is a string containing the base64url-encoded value of the request body. In this case the body content can be compressed or chunked if this is correctly reflected in the Transfer-Encoding header.

This needs to be fixed here: https://github.com/microsoftgraph/msgraph-sdk-dotnet-core/blob/dev/src/Microsoft.Graph.Core/Requests/Content/BatchRequestContent.cs#L147 for requests and we will also need to fix this for responses. The rules for body are the same for responses as requests.

AB#7193