ContentType is a String property that specifies the MIME media type and subtype for the message part, as described in the Internet standards documents: 

RFC 2045 - Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies  

RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types  

RFC 2047 - Multipurpose Internet Mail Extensions (MIME) Part Three: Message Header Extensions for Non-ASCII Text  

ContentType describes the data contained in the message part to allow a user agent to pick an appropriate mechanism for presenting the data. ContentType specifies the nature of the data in the body of an entity by giving media type and subtype identifiers, and by providing auxiliary information that may be required for certain media types. 

After the media type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute=value notation. The ordering of parameters is not significant. 

Use ContentTransfer to identify the encoding scheme for the message part.

TIdMessagePart.ContentTransfer 

TIdMimeTable