function ExtractHeaderSubItem(
    const AHeaderLine: String; 
    const ASubItem: String
): String;

String - Content for the header attribute.

ExtractHeaderSubItem is a String function used to extract the header attribute indicted in ASubItem from the RFC message specified in AHeaderLine. 

AHeaderLine contains the value for the RFC message header after the header name and ':' delimiter have been removed. 

ASubItem contains the attribute name including the '=' character (when required). 

The return value is the contents of the header attribute without leading and trailing '"' delimiters up to a ';' in the message header data.

(Delphi) Extracting an attribute value from a message header. 

Result := ExtractHeaderSubItem(AMsg.ContentType, 'CHARSET=');