Adds magic file type detection to Thoughtbot's Paperclip
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Paperclip is a wonderful module by Thoughtbot that lets makes handling attachments in Rails a breeze, except for one minor niggle - file types. It relies on the content type header sent by the browser which is woefully inconsistent. For example, when uploading a zip the possible values sent for content type are:

  • 'application/octet-stream'
  • 'application/x-zip-compressed'
  • 'application/zip'
  • '' (yes empty)

In short you can't rely on Content-Type from the browser to ever have a sensible value.

Thus the purpose of this module: it uses the wonderful libmagic to work out the file type based on the contents of the uploaded file without shelling out to file to do its work.


The ruby-filemagic gem (which this gem uses) depends upon libmagic must be installed to give us file type detection. On Linux this means installing the relevant package:

sudo apt-get install libmagic-dev # Deb
sudo yum install file-devel # rpm
sudo emerge file # gentoo

OSX ships with the file binary but not the libmagic shared library so I recommend installing it via homebrew:

brew install libmagic
# Since this will be in a non-standard place we need to build ruby-filemagic specially
gem install ruby-filemagic -- --with-opt-dir=$(brew list libmagic | head -n1 | xargs dirname)


Include the gem in your Gemfile:

gem 'magic-paperclip'


Its all automatic - magic-paperclip does its trick by just being in the Gemfile (via a Railtie).

When a Paperclip attachment is assigned to its content_type will be detected and will overwrite what the browser sends.


libmagic often returns charset info or similar for files (i.e. text/plain; charset=us-asci) that this module attempts to remove to use just the base type. Supposedly older versions of libmagic might return this in a different format and thus not get removed. If you find an example of this happening please let me know.

Credits & License


This gem was developed for use at DigiResults and pulled out into a reusable gem.

Released under the MIT License

Copyright (c) 2011, Ash Berlin and DigiResults Ltd.