When attachement has been saved in S3, read bytes for mime_magic

This is to retain compatability with mimemagic 0.3.3, which changed
behavior, expecting anything with a read method to accept a buffered
read request.

Author: reedstrm

app/uploaders/asset_uploader.rb

@@ -51,7 +51,7 @@ def mime_type(file)
     when CarrierWave::SanitizedFile
       file.to_file
     when CarrierWave::Storage::Fog::File
-      CarrierWave::Storage::Fog.new(self).retrieve!(File.basename(file.path))
+      file.read
     else
       open(file)
     end