title: Validation failed: Upload file has an extension that does not match its contents
body: <p>I am using paperclip gem to upload files. and my paperclip gem version is paperclip-4.1.1. While uploading a file its throwing </p>
  
  <pre><code>Validation failed: Upload file has an extension that does not match its contents.
  </code></pre>
  
  <p>I am trying to upload a xlsx file. and also i have mentioned that into the model content_type.</p>
  
  <pre><code> validates_attachment_content_type :upload_file, :content_type =&gt; %w(application/msword application/vnd.ms-office application/vnd.ms-excel application/vnd.openxmlformats-officedocument.spreadsheetml.sheet),
                                                 :message =&gt; ', Only XML,EXCEL files are allowed. '
  </code></pre>
  
  <p>I don't know why this error is happening. If you have any idea about this error please share.</p>
  
  <p>Excerpt from log to show validation failure:</p>
  
  <pre><code>Command :: file -b --mime-type '/tmp/5249540099071db4e41e119388e9dd6220140513-24023-1jlg4zy' [paperclip] Content Type Spoof: Filename file_for_bulk_upload1.xlsx (["application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"]), content type discovered from file command: . See documentation to allow this combination. 
  Command :: file -b --mime-type '/tmp/6f19a4f96154ef7ce65db1d585abdb2820140513-24023-tt4u1e' [paperclip] Content Type Spoof: Filename file_for_bulk_upload1.xlsx (["application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"]), content type discovered from file command:
  </code></pre>