Compiled ImageMagick executables are probably located in /usr/local/bin. That might not work out of the box with Paperclip; in that case an error ending with “…not recognized by the ‘identify’ command.” will appear in the log. The solution is to specify that command path in your config file(s):
Paperclip.options[:command_path]="/usr/local/bin"
In case your before_validate_on_* callback returns false, the model instance will be invalid with @foo.errors.empty? being true.