Thumbnail generation fails for PDFs or OpenDocument

[jnachtigall]

This is a follow-up from #384.

For various "supported" files the thumbnail generation often fails. Here is an example file generated using simple-scan (the Ubuntu default scan program): https://seacloud.cc/f/d4fbcb9078/ The PDF is actually fine (interestingly, I also cannot upload this PDF here at github, too. OTHO, thumbnail generation works at seacloud):

$ file just_a_test_scanned_pdf.pdf 
just_a_test_scanned_pdf.pdf: PDF document, version 1.3

But #384 nonetheless cannot created a thumbnail.

For OpenDocument it sometimes fails and sometimes works (I could maybe send you files privately)