Returns the number of bytes that will be written to {@code sink} in a call to {@link #writeTo}, or -1 if that count is unknown.
Writes the content of this request to {@code sink}.
Returns a new request body that transmits {@code content}. If {@code contentType} is non-null and lacks a charset, this will use UTF-8.
Returns a new request body that transmits {@code content}.
Returns a new request body that transmits the content of {@code file}.
A body content that can be sent or received with an HTTP message.