In an XML file with an element called <file> DATA </file>. DATA can be a BLOB or a filename with a PATH specification.

You can't. What happens when the BLOB just contains a filename? Your DTD isn't too smart. You should think of doing one of:

- provide an attribute which distinguishes the two cases.

- better still, store them in two different kinds of element.

If this really is a BLOB (and not a CLOB) then the data will - hopefully - only be included in a CDATA element anyway. The DOM allows you to distinguish between CDATA and other text nodes.