ContentDisposition is a String property that represents the value for the Internet Message header "Content-Disposition", as described in the Internet Standards document: 

RFC 2183 - Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field  

ContentDisposition indicates presentation information for the message and can contain values and optional parameters. 

ContentDisposition can contain the value "inline" for an Internet Message where the body part should be display immediately, and in the order in which it occurs. 

ContentDisposition can contain the value "attachment" followed by additional parameters to indicate that the message part is separate from the main body of the message. The parameter "filename" can be used to suggest a filename where the message part is detached and stored in a separate file. Other parameters can include "creation-date", "modification-date", "read-date", and "size". 

ContentDisposition refers to the presentation and storage parameters for the entire message. Use TIdAttachment.ContentDisposition to access values for a given attachment in MessageParts.

TIdMessage.MessageParts 

TIdAttachment.ContentDisposition