Forms the content type for data fields in a boundary marker for multipart form data.

sContentTypePlaceHolder = 'Content-Type: %s' + crlf + crlf;

sContentTypePlaceHolder is a String constant used to form the content type header in a boundary marker for multipart form data fields. sContentTypePlaceHolder is used with other constants, like sContentDisposition and sFileNamePlaceHolder, to form the complete boundary marker.