A map representing the parameters of the media type.
A data URI may contain parameters between the MIME type and the
data. This converts these parameters to a map from parameter name
to parameter value.
The map only contains parameters that actually occur in the URI.
The charset parameter has a default value even if it doesn't occur
in the URI, which is reflected by the charset getter. This means that
charset may return a value even if parameters["charset"] is null.
If the values contain non-ASCII values or percent escapes, they default to being decoded as UTF-8.