freedesktop.org.xml.in have wrong entry about DICOM. It have wrong mime-type
(image/x-dcm instead of application/dicom) and wrong magic.
Acording to DICOM specification (part 10 page 20-21
http://medical.nema.org/dicom/2003/03_10PU.PDF ) and RFC 3240 (Digital Imaging
and Communications in Medicine (DICOM) - Application/dicom MIME Sub-type
Registration)
it must look like this:
<mime-type type="application/dicom">
  <_comment>Digital Imaging and Communications in Medicine image</_comment>
  <magic priority="50">
    <match type="string" value="DICM" offset="128"/>
  </magic>
  <glob pattern="*.dcm"/>
</mime-type>

Please add this entry and remove <mime-type type="image/x-dcm"> entry.