The MIME type of the data URI.
A data URI consists of a "media type" followed by data. The media type starts with a MIME type and can be followed by extra parameters.
This data URI has the media type
text/plain;charset=utf-8, which is the
text/plain with the parameter
charset with value
See RFC 2045 for more detail.
If the first part of the data URI is empty, it defaults to