ype</code>, <code>subtype</code>, <code>attribute</code> and <code>value</code> are specified in RFC-2045,
and <code>data</code> is a sequence of URI-characters (RFC-2396 <code>uric</code>).</p>
<p>This means that all the characters must be ASCII, but the URI may contain
percent-escapes for non-ASCII byte values that need an interpretation
to be converted to the corresponding string.</p>
<p>Parsing checks that Base64 encoded data is valid, and it normalizes it
to use the default Base64 alphabet and to use p