When using `contentType` for `requestBody` the it is shown Example values (default)

When using the following:

```js
const SignUpResponse = {
message: "User Added!"
}

export class SignUp extends OpenAPIRoute {
static schema = {
tags: ["Users"],
summary: "User Sign Up",
requestBody: {
contentType: 'text/plain',
firstname: new Str({
description: "User Firstname",
required: true,
example: "Ashish"
}),
lastname: new Str({
description: "User Lastname",
required: true,
example: "Jullia"
}),
email: new Str({
description: "User Email",
required: true,
example: "username@example.com"
}),
password: new Str({
description: "User Password",
required: true,
example: "As@121212121212"
})
},
responses: {
"200": {
contentType: 'application/json',
schema: {
Response: SignUpResponse,
},
},
},
};

async handle(
request,
env,
context,
data
) {
// Retrieve the validated slug
const { firstname, lastname, email, password } = data.body

// console.log(isValidEmail(email))

return {
message: "User Added!"
};
}
}
```

It shows:


I know I'm missing something but how to correctly pass this `contentType` and where?