title: Paperclip validation error on rails
body: <p>I'm using paperclip to upload excel files. There's a file that i can upload from my mac and windows using any browser. But when we try my co-worker's computer it says that file is not valid for uploading.</p>
  
  <p>Is that even possible? I mean i thought validation process works on server. How can we get different results with the same file, same server, same browser? </p>
  
  <p>Here's the validation code which i use on my model. </p>
  
  <p><code>
      has_attached_file :attachment
      validates_attachment_content_type :attachment,  content_type:['application/vnd.ms-excel', 'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet']
  </code></p>
  
  <p>What do u guys think about why its acting like that? </p>