When including unparsed content, the applications processing the data need
to know something about the format of the data included, in order to process
it correctly. For this we need to create a
<!NOTATION> entry. So for
example, we might have:
<!NOTATION jpg SYSTEM "image/jpeg">
<!NOTATION svg SYSTEM "image/svg-xml">
to allow us to use two different formats for graphical data. Note that the second, SVG, is also an XML document, though we don't want to parse it - it should be passed directly to the application.
The value given for each type of file is called a MIME-type, and is a standard code that most web browsers and data processing systems can use. There are numerous lists of MIME-types on the web; the most authoritative is at IANA, and a slightly friendlier list can be seen at W3Schools.