Which XSLT 1.0 processor do you use? Xalan Java supports
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="1.0"
xmlns:redirect="http://xml.apache.org/xalan/redirect"
extension-element-prefixes="redirect"
exclude-result-prefixes="redirect">
<xsl:template match="/Envelope/Notification/Data[not(Input/*)]">
<redirect:write select="concat(substring(Input, 1, 6), '.xml')">
<xsl:copy-of select="Output"/>
</redirect:write>
</xsl:template>
<xsl:template match="/Envelope/Notification/Data[Input/*]">
<redirect:write select="concat(local-name(Input/*), '.xml')">
<xsl:copy-of select="Output"/>
</redirect:write>
</xsl:template>
</xsl:stylesheet>