[imp] pdf image preview

[Liam Hoekenga]

I'm getting ready to roll out the latest release of Horde (3.3.4) /IMP  
(4.3.4), and I'm trying to get the PDF thumbnailing to work.  I've got  
the imagemagick php extension installed (imagick), and I've pointed  
horde to the location of imagemagick's "covert" utility, but when I  
receive a message with a PDF attachment, it just says..

     A PDF file named example.pdf is attached to this message.
     A thumbnail is below.

     View Attachment

Where "View Attachment" is link to the file.  Looking at the source  
for horde/imp/lib/MIME/Viewer/pdf.php, it looks like the "View  
Attachment" should be the ALT tag for the thumbnail image.  Thumbnails  
for image attachments work ok.

I'm pretty sure that my imagemagick installation is working.  ldd  
doesn't show any missing libraries, and phpinfo() shows that the  
imagick extension is loaded.

What am I missing?

Liam