Specifies the mime type of the encrypted data.
MimeType is an optional (advisory) attribute which describes the media type of the data which is 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.