ContentDisposition

Creating a properly encoded and escaped standards-compliant HTTP Content-Disposition header for potential filenames with special characters is surprisingly confusing.

This ruby gem does that and only that, in a single 50-line file with no dependencies. It's code is shamelessly extracted and adapted from Rails' ActionDispatch::HTTP::ContentDisposition class.

Before we proceed with the usage guide, first a bit of explanation what is the Content-Disposition header. The Content-Disposition response header specifies the behaviour of the web browser when opening a URL.

The inline disposition will display the content "inline", which means that known MIME types from the Content-Type response header are displayed inside the browser, while unknown MIME types will be immediately downloaded.

Content-Disposition: inline

The attachment disposition will tell the browser to always download the content, regardless of the MIME type.

Content-Disposition: attachment

When the content is downloaded, by default the filename will be last URL segment. This can be changed via the filename parameter:

Content-Disposition: attachment; filename="image.jpg"

To support old browsers, the filename should be the ASCII version of the filename, while the filename* parameter can be used for the full filename with any potential UTF-8 characters. Special characters from the filename need to be URL-encoded in both parameters.

Add this line to your application's Gemfile:

gem "content_disposition", "~> 1.0"

And then execute:

$ bundle

Or install it yourself as:

$ gem install content_disposition

require "content_disposition"

ContentDisposition.format(disposition: "inline", filename: "racecar.jpg")
# => "inline; filename=\"racecar.jpg\"; filename*=UTF-8''racecar.jpg"

A proper content-disposition value for non-ascii filenames has a pure-ascii as well as an ascii component. By default the filename will be turned into ascii by replacing any non-ascii chars with '?' (which is then properly percent-escaped to %3F in output).

ContentDisposition.format(disposition: "attachment", filename: "råcëçâr.jpg")
# => "attachment; filename=\"r%3Fc%3F%3F%3Fr.jpg\"; filename*=UTF-8''r%C3%A5c%C3%AB%C3%A7%C3%A2r.jpg"

But you can pass in your own proc to do it however you want. If you have a dependency on the i18n gem, and want to do it just like Rails:

ContentDisposition.format(
  disposition: "attachment",
  filename: "råcëçâr.jpg",
  to_ascii: ->(filename) { I18n.transliterate(filename) }
)
# => "attachment; filename=\"racecar.jpg\"; filename*=UTF-8''r%C3%A5c%C3%AB%C3%A7%C3%A2r.jpg"

You can also configure .to_ascii globally for any invocation:

ContentDisposition.to_ascii = ->(filename) { I18n.transliterate(filename) }

The .format method is aliased to .call, so you can do:

ContentDisposition.(disposition: "inline", filename: "råcëçâr.jpg")
# => "inline; filename=\"r%3Fc%3F%3F%3Fr.jpg\"; filename*=UTF-8''r%C3%A5c%C3%AB%C3%A7%C3%A2r.jpg"

There are also .attachment and .inline shorthands:

ContentDisposition.attachment("racecar.jpg")
# => "attachment; filename=\"racecar.jpg\"; filename*=UTF-8''racecar.jpg"
ContentDisposition.inline("racecar.jpg")
# => "inline; filename=\"racecar.jpg\"; filename*=UTF-8''racecar.jpg"

You can also create a ContentDisposition instance to build your own Content-Disposition header.

content_disposition = ContentDisposition.new(
  disposition: "attachment",
  filename:    "råcëçâr.jpg",
)

content_disposition.disposition
# => "attachment"
content_disposition.filename
# => "råcëçâr.jpg"

content_disposition.ascii_filename
# => "filename=\"r%3Fc%3F%3F%3Fr.jpg\""
content_disposition.utf8_filename
# => "filename*=UTF-8''r%C3%A5c%C3%AB%C3%A7%C3%A2r.jpg"

content_disposition.to_s
# => "attachment; filename=\"r%3Fc%3F%3F%3Fr.jpg\"; filename*=UTF-8''r%C3%A5c%C3%AB%C3%A7%C3%A2r.jpg"

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Bug reports and pull requests are welcome on GitHub at https://github.com/shrinerb/content_disposition.

The gem is available as open source under the terms of the MIT License.