Retrieves the message part specified by PartID.
The 'callback' parameter specifies a function which will be called when the operation completes (or an error is encountered). If the 'callback' parameter is not specified, then the method will block and will not return until the operation completes (or an error is encountered).
The callback for this method is defined as:
'err' is the error that occurred. If there was no error, then 'err' is 'null'.
'err' has 2 properties which hold detailed information:
This method retrieves the contents of the message part specified by the PartId property of the message specified by the MessageSet property. If the LocalFile property contains a file name, the message part is stored in LocalFile, and the Transfer events denote the progress. If the LocalFile property contains an empty string, the message part is stored in the MessageText property, as well as provided through the Transfer event.
The part identifiers of each message part (if any) are provided through the corresponding MessagePart events.