DOCBOOK-APPS: problem w/tiff images and fo output

using v1.49 of the xsl stylesheets

I have the following figure defined:

<figure><title>CCSmartWheel</title>
    <mediaobject>
      <imageobject>
	<imagedata fileref="images/tiff/smartwheel.tiff" format="TIFF"/>
      </imageobject>
      <textobject>
	<phrase>CCSmartWheel</phrase>
      </textobject>
    </mediaobject>
</figure>

When output to fo no <fo:external-graphic> element is generated.  If I replace the tiff spec with one for a png it works.
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-s