lements <a href="../../../jakarta/mail/EncodingAware.html" title="interface in jakarta.mail"><code>EncodingAware</code></a>, ask it
 what encoding to use.  If it returns non-null, return that value.
 <li>
 If the primary type of this datasource is "text" and if all
 the bytes in its input stream are US-ASCII, then the encoding
 is "7bit". If more than half of the bytes are non-US-ASCII, then
 the encoding is "base64". If less than half of the bytes are
 non-US-ASCII, then the encoding is "quoted-printable".