Is your feature request related to a problem? Please describe.
Currently in the demo app, typescript complains about this method access:

Property 'get' does not exist on type 'BaseBody'.
  Property 'get' does not exist on type 'string'.

Describe the solution you'd like
The export statement should probably start with

export const post: RequestHandler<any, ReadOnlyFormData>

But that requires ReadOnlyFormData being able to be imported

Describe alternatives you've considered

How important is this feature to you?

Additional context