I decided to go the route that John suggested, and use the EXPath File

module. John was right, it was fairly easy to implement the desired
functionality using the EXPath File module. Below is what I created. It seems
to work fine. If you see any potential problems in it, or ways to improve it,
please let me know.  /Roger

<xsl:function name="f:remove-last-slash-filename-from-URI"

<xsl:param name="uri" as="xs:string"/>

<xsl:value-of

</xsl:function>

I think with pure XPath I tend to use
  tokenize($uri, '/')[last()]
here to select the file name.

But the reverse-string(substring-after looks a bit too complicated to me
with access to the tokenize function.