[xsl] Determine file extension of path stored in variable

[John <john-xsl-list@xxxxxxxx>]

What is the best way to determine the extension of a file when the 
webroot-relative path is stored in a variable, assuming the path may 
contain any number of "." characters (so I can't use substring-after)?  
For instance if $lcpath contains /images/dir.path/file.png how do I set 
$ext to be "png"?  Below is the best I have been able to come up with - 
suggestions greatly appreciated.

<xsl:variable name="lcpath" select="/images/dir.path/file.png" />

<xsl:variable name="pos">
 <xsl:choose>
   <xsl:when test="substring( $lcpath, string-length( $lcpath ) - 5, 1 
) = '.'">4</xsl:when>
   <xsl:when test="substring( $lcpath, string-length( $lcpath ) - 4, 1 
) = '.'">3</xsl:when>
   <xsl:when test="substring( $lcpath, string-length( $lcpath ) - 3, 1 
) = '.'">2</xsl:when>
   <xsl:when test="substring( $lcpath, string-length( $lcpath ) - 2, 1 
) = '.'">1</xsl:when>
   <xsl:otherwise>0</xsl:otherwise>
 </xsl:choose>
</xsl:variable>
<xsl:variable name="ext" select="substring( $lcpath, string-length( 
$lcpath ) - $pos )" />