Just a quick post on another problem I ran into while setting up PaperClip. When attempting to submit a form WITHOUT an image I received the following error message:

 
Assets asset /tmp/stream20120218-22611-dgnkr-0.exe is not recognized by the 'identify' command.

Apparently the most common cause of this issue is one of two things:

#1: You don’t have RMagick installed
The easiest fix for this is to simply install the rmagick gem. Simply add rmagick to your Gemfile and run bundle install. Restart your server and you should be ready to go.

#2: Your command_path isn’t set correctly
Run the following command from your console:

chris@chris-VirtualBox:~/site$ which identify
/usr/bin/identify

Then add the path that is returned to your environment settings:

Paperclip.options[:command_path] = "/usr/bin"

#3: If all else fails

If none of the above seem to be causing your issue, try adding the following to your model:

 def asset?
    !(asset_content_type =~ /^image.*/).nil?
  end

This will simply prevent the file being processed if it’s not an image, avoiding the issue all together. Hopefully this’ll help someone out, let me know if you come across any other solutions.

UPDATE:
Apparently a more recent cause of this issue is an update to the cocaine gem, rolling back to version 0.3.2 appears to resolve the issue. Thanks to Ahmed and Fabrice for bringing it to my attention:
https://github.com/thoughtbot/paperclip/issues/1038
http://stackoverflow.com/a/12760881/522859