How to accept requests that do not use json encoding?

[khanetor]

I have a request like this

POST /api/?method=some-method&encrypted=true HTTP/1.1
Host: axis.com
Content-Type: application/x-www-form-urlencoded
Content-Length: NNN
some_long_string_without_double_quotes_here

Due to the origin of this request, nothing can be done to change it to use json content-type. Regardless, I have a schema like this

@simpleRestJson
service CameraService {
    version: "1.0.0",
    operations: [Report]
}

@http(method: "POST", uri: "/api", code: 200)
operation Report {
    input: CameraQuery,
    output: AxisTimestamp
}

structure CameraQuery {
    @required
    @httpQuery("method")
    method: String,

    @required
    @httpQuery("encrypted")
    encrypted: Boolean,

    @required
    @httpPayload
    payload: String
}

When I submit the request above with double quotes around the body, then it works. But if I remove the double quotes, then I get 500, as expected because the body does not confirm to json format. I then tried changing the payload to Blob instead of String, and I get 500 error regardless.

How should I handle this case?