You can use the org.eclipse.wst.xml.core.catalogContributions extension point (this requires that you have the Eclipse Web Tools (WST) component installed.
The following is the contribution of the org.eclipse.wst.xsd.core plugin:
<extension point="org.eclipse.wst.xml.core.catalogContributions"> <catalogContribution id="default"> <uri name="http://www.w3.org/2001/XMLSchema" uri="platform:/plugin/org.eclipse.xsd/cache/www.w3.org/2001/XMLSchema.xsd" /> <system systemId="http://www.w3.org/2001/xml.xsd" uri="platform:/plugin/org.eclipse.xsd/cache/www.w3.org/2001/xml.xsd"/> </catalogContribution> </extension>
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