“Mime type of document could not be determined” Error on Apollo Server

I was given a link for a graphQL server with already built in queries and mutations.

When I tried to run a mutation, I got the following response:

{
  "data": {},
  "errors": [
    {
      "message": "Mime type of document could not be determined"
    }
  ]
}

What could be the problem and how can i fix it?