.pdf thumbnails do not work in redmine 4.1.1; .txt icons do not work

[GaryAitken]

redmine 4.1.1, ubuntu 18.04
pdf -- thumbnails don't seem to work, but icon does show up if switched on instead.
txt -- neither thumbnails nor icons work

[hermannkm]

Same problem for pdf on 4.2.2, Ubuntu 20.04, ImageMagick 6.9.10-23 Q16 x86_64 20190101.
Thumbnails are empty after some short pause -- something seems to happen but no result is produced.
Log shows

Creating thumbnail failed (pid 235720 exit 1):
Command: 'convert' '/var/www/redmine-4.2/files/2021/08/210808080826_dfu_SR8200N.pdf[0]' -thumbnail '200x200>' 'png:/var/www/redmine-4.2/tmp/thumbnails/94db495653ada2aa56003cb489e0425a8b

After some searching I found a solution (not sure about security implications) e.g. here: https://askubuntu.com/questions/1127260/imagemagick-convert-not-allowed
and solved the problem by commenting line 94 in /etc/ImageMagick-6/policy.xml:
<!-- <policy domain="coder" rights="none" pattern="PDF" /> -->
Nice thumbnails instantly for PDFs.