GiraffeMediaTypeConfig.fs

let customJsonContentType mimeType data next (ctx : HttpContext) =
    sprintf "%s; charset=utf-8" mimeType |> ctx.SetContentType
    let serializer = ctx.GetJsonSerializer()
    serializer.SerializeToBytes data
    |> ctx.WriteBytesAsync

type CustomNegotiationConfig (baseConfig : INegotiationConfig) =
    interface INegotiationConfig with
        member __.UnacceptableHandler =
            baseConfig.UnacceptableHandler
        member __.Rules =
            dict [
                "*/*" , json
                "application/json" , json
                "application/vnd.chamook.mini-colours+json",
                customJsonContentType "application/vnd.chamook.mini-colours+json"
                ]

let configureServices (services : IServiceCollection) =
    services.AddGiraffe() |> ignore

    CustomNegotiationConfig(DefaultNegotiationConfig())
    |> services.AddSingleton<INegotiationConfig>
    |> ignore