Designing a Form Serializer

Who decides the boundary in a multipart/form-data request?

An Form Serializer builds a string out of the successful HTML Form
controls. The string will be passed to an AsyncRequest adapter for
XMLHttpRequest.send( data )

Form::getMultipartFormData::string[]
AsyncRequest::send( data::string );

The multipart-form/data is where I am having difficulty. A boundary
separates field data in the request body. The boundary exists in two
places: The data and the HTTP header request.

I want to keep the two decoupled.

Where should the boundary get created: Form or AsyncRequest?

1) AsyncRequest::send:
will have to do some extra work joining form data on the boundary.
2) Form::getMultipartFormDataString
will have to provide some way for the boundary to be obtained.

Where should the boundary get created? The AsyncRequest::send, or the
Form::getMultipartFormDataString?

Garrett