title: content-type validation with paperclip fails
body: <p>I'm trying to use paperclip to upload .xm files which should have the mime type <code>audio/x-mod</code>.
  I configured paperclip to allow this configuration by setting the following options:</p>
  
  <pre><code>Paperclip.options[:content_type_mappings] = {
      xm: "audio/x-mod"
  }
  </code></pre>
  
  <p>The validation for the attachment field looks like this:</p>
  
  <pre><code>validates_attachment :song, presence: true,
    content_type: { content_type: ["audio/x-mod"] },
    size: { in: 0..128.kilobytes }
  </code></pre>
  
  <p>Whenever I try to upload a .xm file that has the mime type <code>audio/x-mod</code> paperclip, I get the error <code>Song content type is invalid</code>.
  It works when I specify the valid content types as <code>[/.+/]</code>.</p>
  
  <p>Have I missed anything? Is this maybe a bug in how paperclip checks the content type? Is there some way to see what paperclip thinks the content type of the file is?</p>
  
  <pre><code>Started POST "/mods" for 127.0.0.1 at 2014-08-02 11:28:56 +0200
  Processing by ModsController#create as HTML
    Parameters: {"utf8"=&gt;"✓", "authenticity_token"=&gt;"FhBvyd8jOapcjONk8kyOgGE/oOZPA+sDBJxr/w3zUG0=", "mod"=&gt;{"title"=&gt;"Girl Next Door", "release(1i)"=&gt;"2014", "release(2i)"=&gt;"8", "release(3i)"=&gt;"2", "song"=&gt;#&lt;ActionDispatch::Http::UploadedFile:0x00000000e763c8 @tempfile=#&lt;Tempfile:/tmp/RackMultipart20140802-1742-wujc7n&gt;, @original_filename="Wiklund_-_Girl_next_door.xm", @content_type="audio/x-xm", @headers="Content-Disposition: form-data; name=\"mod[song]\"; filename=\"Wiklund_-_Girl_next_door.xm\"\r\nContent-Type: audio/x-xm\r\n"&gt;}, "commit"=&gt;"Save"}
  Command :: file -b --mime '/tmp/d437374435a48a211b1f7b9e585c4c2d20140802-1742-1259ddt.xm'
     (0.1ms)  begin transaction
  Command :: file -b --mime '/tmp/d437374435a48a211b1f7b9e585c4c2d20140802-1742-1e5e5sn.xm'
  </code></pre>