I guess in xslt you could use "contains()" or "ends-with()" to check for the extensions, but this can also be done just in css. 

<xsl:if test="contains($fileName,'.pdf')">add pdf icon</xsl:if>

or

<xsl:if test="ends-with($fileName,'.pdf')">add pdf icon</xsl:if>

Forget about ie6 and try this

a[href$='.pdf'] {
display:inline-block;
padding-left:20px;
line-height:18px;
background:transparent url(images/pdficon.gif) center left no-repeat;
}

a[href$='.xls'], a[href$='.csv'], a[href$='.xlt'], a[href$='.xlw'] {
display:inline-block;
padding-left:20px;
line-height:18px;
background:transparent url(images/excelicon.gif) center left no-repeat;
}

a[href$='.ppt'], a[href$='.pps'] {
display:inline-block;
padding-left:20px;
line-height:18px;
background:transparent url(images/ppticon.gif) center left no-repeat;
}

a[href$='.doc'], a[href$='.rtf'], a[href$='.txt'], a[href$='.wps'] {
display:inline-block;
padding-left:20px;
line-height:18px;
background:transparent url(images/wordicon.gif) center left no-repeat;
}

a[href$='.zip'], a[href$='.gzip'], a[href$='.rar'] {
display:inline-block;
padding-left:20px;
line-height:18px;
background:transparent url(images/zipicon.gif) center left no-repeat;
}