If the path to ImageMagick in the attachments pane of the Admin CP doesn't end in a trailing slash (i.e. is "/usr/bin" rather than "/usr/bin/") then ImageMagick isn't used to create the thumbnails for attachments and GD is used instead. Looking at the code, there is no check to see if a trailing slash should be added. Therefore if one isn't present, the command that gets executed is something like:

/usr/binconvert -quality 85 -antialias ...

Which, of course, doesn't work!

Also... it would be handy if there was a way to search this Bug Tracker...