<extension point="org.eclipse.ui.editors">
    <editor
        class="com.hazem.msxmleditor.ui.MSXMLEditor"
        default="true"
        extensions="msxml"
        id="com.hazem.msxmleditor.ui.MSXMLEditor"
        name="MS Editor"/>
</extension>

<msxml version="1.3">
    ...
</msxml>

import org.xml.sax.Attributes;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

public class XMLRootVersionHandler extends DefaultHandler {

	String localQualifiedTagName = null;
	String localAttributeName = null;
	String localVersion = null;
	
	boolean nextIsVersion = false;
	
	public void startElement(String uri, String localName, String qname, Attributes attr) throws SAXException {
		if(qname != null && qname.equals(localQualifiedTagName))
		{
			if(localAttributeName==null){
				nextIsVersion = true;
			}else{
				localVersion = attr.getValue(localAttributeName);
				throw new XMLRootVersionTerminatorException();
			}
		}
	}
	public void characters(char[] characters, int start, int length) throws SAXException  {
		if(nextIsVersion){
			localVersion = new String(characters, start, length);
			throw new XMLRootVersionTerminatorException();
		}
	}
	
	public String getVersion(){
		return localVersion;		
	}

	public void setStringsToSearch(String qualifiedTagName, String attributeName){
		localQualifiedTagName = qualifiedTagName;
		localAttributeName = attributeName;
	}
}

import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;
import java.util.Hashtable;
import org.eclipse.core.internal.content.ContentMessages;
import org.eclipse.core.runtime.CoreException;
import org.eclipse.core.runtime.IConfigurationElement;
import org.eclipse.core.runtime.IExecutableExtension;
import org.eclipse.core.runtime.IStatus;
import org.eclipse.core.runtime.Status;
import org.eclipse.core.runtime.content.IContentDescription;
import org.eclipse.core.runtime.content.XMLContentDescriber;
import org.eclipse.osgi.util.NLS;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.XMLReaderFactory;

@SuppressWarnings({"rawtypes", "unchecked" })
public class XMLRootVersionDescriber extends XMLContentDescriber implements IExecutableExtension {

	Hashtable<String, String> describerParameters = null;	

	static final String qualifiedTagNameParameter = "qualifiedTagName";
	static final String versionParameter = "version";
	static final String attributeNameParameter = "attributeName";
			
	/* (Intentionally not included in javadoc)
	 * @see IContentDescriber#describe(InputStream, IContentDescription)
	 */
	public int describe(InputStream contents, IContentDescription description) throws IOException {

		// super.describe will have consumed some chars, need to rewind		
		contents.reset();
		// Check to see if we matched our criteria.		
		int checkCriteria = checkCriteria(new InputSource(contents));
		System.out.println("v2 Return (1) : " + checkCriteria);
		return checkCriteria;
	}

	/* (Intentionally not included in javadoc)
	 * @see IContentDescriber#describe(Reader, IContentDescription)
	 */
	public int describe(Reader contents, IContentDescription description) throws IOException {
		int checkCriteria = checkCriteria(new InputSource(contents));
		System.out.println("v2 Return (2) : " + checkCriteria);
		return checkCriteria;
	}
		
	private int checkCriteria(InputSource contents) throws IOException {
        XMLRootVersionHandler handler = new XMLRootVersionHandler();
        
		try {
			
	        XMLReader saxReader = XMLReaderFactory.createXMLReader();
	        handler.setStringsToSearch(
	        		describerParameters.get(qualifiedTagNameParameter),
	        		describerParameters.get(attributeNameParameter)
	        		);
			saxReader.setContentHandler(handler);
			saxReader.parse(contents);
			
		}catch (XMLRootVersionTerminatorException e){
			//ok
			String version = (String) handler.getVersion();
			if (version != null && version.equals(describerParameters.get(versionParameter))){
				return VALID;
			}
		}catch (SAXException e) {
			// we may be handed any kind of contents... it is normal we fail to parse
			return INVALID;
		}catch (Exception e) {
			// we may be handed any kind of contents... it is normal we fail to parse
			return INVALID;
		}
		return INVALID;
	}
	
	/* (Intentionally not included in javadoc)
	 * @see IExecutableExtension#setInitializationData
	 */
	public void setInitializationData(final IConfigurationElement config, final String propertyName, final Object data) throws CoreException {
		
		if (data instanceof Hashtable) {
			if(((Hashtable) data).get(qualifiedTagNameParameter) != null && ((Hashtable) data).get(versionParameter) != null ){
				describerParameters = (Hashtable) data;
			}
		}

		if (describerParameters == null) {
			String message = NLS.bind(ContentMessages.content_badInitializationData, XMLRootVersionDescriber.class.getName());
			throw new CoreException(new Status(IStatus.ERROR, ContentMessages.OWNER_NAME, 0, message, null));
		}
	}
}

import org.xml.sax.SAXException;

public class XMLRootVersionTerminatorException extends SAXException {

	private static final long serialVersionUID = 4280571238180794197L;

}

<plugin>
  <extension point="org.eclipse.ui.editors">
    <editor
          class="path.to.editor.ui.MSXMLEditor"
          default="true"
          id="path.to.editor.ui.MSXMLEditor"
          name="MSXML Editor">
       <contentTypeBinding
             contentTypeId="path.to.editor.content-type">
       </contentTypeBinding>
    </editor>
  </extension>
  <extension
        point="org.eclipse.core.contenttype.contentTypes">
     <content-type
           base-type="org.eclipse.core.runtime.xml"
           file-extensions="msxml"
           id="path.to.editor.content-type"
           name="MSXML File"
           priority="normal">
       <describer class="path.to.editor.XMLRootVersionDescriber">
        <parameter
              name="qualifiedTagName"
              value="msxml">
        </parameter>
        <parameter
              name="attributeName"
              value="version">
        </parameter>
        <parameter
              name="version"
              value="1.3">
        </parameter>
       </describer>
     </content-type>
  </extension>
</plugin>