Yep. This is valid

    QUERY /graphql
    Content-Type: text/graphql
    {too lazy to write valid gql}

so is this

    QUERY /elasticsearch
    Content-Type: application/json
    {"name": "alice"}

Even this

    QUERY /my-ai-image-gen
    Content-Type: text/plain
    Draw me a picture of a cat

It's entirely up to the software to decide how to handle the request.