title: S3 direct upload restrict file type
body: <p>I generate a presigned post like this : (Note: I am using aws-sdk v1)</p>
  
  <pre><code> @s3_direct_post = S3_BUCKET.presigned_post(
                              key: "uploads/#{SecureRandom.uuid}/${filename}", 
                              success_action_status: '201',
                              acl: 'public-read',
                              content_type: 'image/png'
                              )
  </code></pre>
  
  <p>I would like to restrict user uploads to only png files, however this doesn't works and it allow all kind off files.</p>