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.
Example:
data:text/plain;charset=utf-8,Hello%20World!
This data URI has the media type text/plain;charset=utf-8, which is the
MIME type text/plain with the parameter charset with value utf-8.
See RFC 2045 for more detail.
If the first part of the data URI is empty, it defaults to text/plain.