Specifies the mime type of the encrypted data.
MimeType describes the media type of the data which has been encrypted. The value of this attribute is a string with values defined by MIME specification (RFC 2045). For example, if the data that is encrypted is a base64 encoded PNG, the transfer Encoding may be specified as 'http://www.w3.org/2000/09/xmldsig#base64' and the MimeType as 'image/png'. This attribute is purely advisory; no validation of the MimeType information is required and it does not indicate the encryption application must do any additional processing.