The mimetype that is used currently can not be changed.
The mimetype will be "application/pdf" for PDF documents, "application/vnd.ms-xpsdocument" for XPS documents and "text/plain" for PlainText documents. If Format is not recognised, the propery will return null.
Target Platforms: Windows 10, Windows 8.1, Windows 8, Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2