content-type values when using collection() and metadata=yes

So I have a folder structure that contains DITA XML and some other stuff
(images, etc.).  If I use the "select content by metadata" example (
https://www.saxonica.com/html/documentation/sourcedocs/collections.html) and
use a content type of "application/xml", I get nothing.

If I use:

<xsl:variable as="document-node()+" name="contentSet"
      select="for $m in (($sourceDir ||
'?recurse=yes;on-error=warning;metadata=yes' ) => collection())
      return if ($m?content-type='application/unknown') then $m?fetch()
else ()" />

I get the expected XML documents.

Is there a way to explain new content types to collection()?

Thanks!
Graydon

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