title: After replacing Rmagick with Minimagick, gets 'identify.im6: no decode delegate for this image format' error
body: <p>In my Rails app, I just removed <code>Rmagick</code> and added <code>Minimagick</code>. Rmagick was too heavy. Things worked fine with Rmagick but in minimagick I get the following error :</p>
  
  <pre><code>MiniMagick::Invalid: `identify /tmp/mini_magick20150518-10411-1qz42x8` failed with error:
  identify.im6: no decode delegate for this image format `/tmp/mini_magick20150518-10411-1qz42x8' @ error/constitute.c/ReadImage/544.
  from /home/pubudu/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/mini_magick-4.2.4/lib/mini_magick/image.rb:200:in `rescue in validate!'
  </code></pre>
  
  <p>The file url :</p>
  
  <pre><code>[8] pry(main)&gt; file.file.file
  =&gt; "/home/pubudu/Projects/istockseller/public/store/photos/thpubs/istockseller-13642162-wallhaven-61582.jpg"
  </code></pre>
  
  <p>The code im trying to run :</p>
  
  <pre><code>MiniMagick::Image.read(file.file.file).first
  </code></pre>