Returns the default content type for the payload. Called when
{@link #toMessage(Object, MessageHeaders)} is invoked without message headers or
without a content type header.
<p>By default, this returns the first element of the {@link #getSupportedMimeTypes()
supportedMimeTypes}, if any. Can be overridden in sub-classes.
@param payload the payload being converted to message
@return the content type, or {@code null} if not known
Returns the default content type for the payload. Called when {@link #toMessage(Object, MessageHeaders)} is invoked without message headers or without a content type header. <p>By default, this returns the first element of the {@link #getSupportedMimeTypes() supportedMimeTypes}, if any. Can be overridden in sub-classes. @param payload the payload being converted to message @return the content type, or {@code null} if not known