mime-types for Ruby











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The mime-types library provides a library and registry for information about MIME content type definitions. It can be used to determine defined filename extensions for MIME types, or to use filename extensions to look up the likely MIME type definitions.

Version 3.0 is a major release that requires Ruby 2.0 compatibility and removes deprecated functions. The columnar registry format introduced in 2.6 has been made the primary format; the registry data has been extracted from this library and put into mime-types-data. Additionally, mime-types is now licensed exclusively under the MIT licence and there is a code of conduct in effect. There are a number of other smaller changes described in the History file.

About MIME Media Types

MIME content types are used in MIME-compliant communications, as in e-mail or HTTP traffic, to indicate the type of content which is transmitted. The mime-types library provides the ability for detailed information about MIME entities (provided as an enumerable collection of MIME::Type objects) to be determined and used. There are many types defined by RFCs and vendors, so the list is long but by definition incomplete; don’t hesitate to add additional type definitions. MIME type definitions found in mime-types are from RFCs, W3C recommendations, the IANA Media Types registry, and user contributions. It conforms to RFCs 2045 and 2231.

mime-types 3.x

Users are encouraged to upgrade to mime-types 3.x as soon as is practical. mime-types 3.x requires Ruby 2.0 compatibility and a simpler licensing scheme.


MIME types are used in MIME entities, as in email or HTTP traffic. It is useful at times to have information available about MIME types (or, inversely, about files). A MIME::Type stores the known information about one MIME type.

require 'mime/types'

plaintext = MIME::Types['text/plain'] # => [ text/plain ]
text = plaintext.first
puts text.media_type            # => 'text'
puts text.sub_type              # => 'plain'

puts text.extensions.join(' ')  # => 'txt asc c cc h hh cpp hpp dat hlp'
puts text.preferred_extension   # => 'txt'
puts text.friendly              # => 'Text Document'
puts text.i18n_key              # => 'text.plain'

puts text.encoding              # => quoted-printable
puts text.default_encoding      # => quoted-printable
puts text.binary?               # => false
puts text.ascii?                # => true
puts text.obsolete?             # => false
puts text.registered?           # => true
puts text.provisional?          # => false
puts text.complete?             # => true

puts text                       # => 'text/plain'

puts text == 'text/plain'       # => true
puts 'text/plain' == text       # => true
puts text == 'text/x-plain'     # => false
puts 'text/x-plain' == text     # => false

puts MIME::Type.simplified('x-appl/x-zip') # => 'x-appl/x-zip'
puts MIME::Type.i18n_key('x-appl/x-zip') # => 'x-appl.x-zip'

puts text.like?('text/x-plain') # => true
puts text.like?(MIME::Type.new('x-text/x-plain')) # => true

puts text.xrefs.inspect # => { "rfc" => [ "rfc2046", "rfc3676", "rfc5147" ] }
puts text.xref_urls # => [ "http://www.iana.org/go/rfc2046",
                    #      "http://www.iana.org/go/rfc3676",
                    #      "http://www.iana.org/go/rfc5147" ]

xtext = MIME::Type.new('x-text/x-plain')
puts xtext.media_type # => 'text'
puts xtext.raw_media_type # => 'x-text'
puts xtext.sub_type # => 'plain'
puts xtext.raw_sub_type # => 'x-plain'
puts xtext.complete? # => false

puts MIME::Types.any? { |type| type.content_type == 'text/plain' } # => true
puts MIME::Types.all?(&:registered?) # => false

# Various string representations of MIME types
qcelp = MIME::Types['audio/QCELP'].first # => audio/QCELP
puts qcelp.content_type         # => 'audio/QCELP'
puts qcelp.simplified           # => 'audio/qcelp'

xwingz = MIME::Types['application/x-Wingz'].first # => application/x-Wingz
puts xwingz.content_type        # => 'application/x-Wingz'
puts xwingz.simplified          # => 'application/x-wingz'

Columnar Store

mime-types uses as its primary registry storage format a columnar storage format reducing the default memory footprint. This is done by selectively loading the data on a per-attribute basis. When the registry is first loaded from the columnar store, only the canonical MIME content type and known extensions and the MIME type will be connected to its loading registry. When other data about the type is required (including preferred_extension, obsolete?, and registered?) that data is loaded from its own column file for all types in the registry.

The load of any column data is performed with a Mutex to ensure that types are updated safely in a multithreaded environment. Benchmarks show that while columnar data loading is slower than the JSON store, it cuts the memory use by a third over the JSON store.

If you prefer to load all the data at once, this can be specified in your application Gemfile as:

gem 'mime-types', require: 'mime/types/full'

Projects that do not use Bundler should require the same:

require 'mime/types/full'

Libraries that use mime-types are discouraged from choosing the JSON store.

For applications and clients that used mime-types 2.6 when the columnar store was introduced, the require used previously will still work through at least version 4 and possibly beyond; it is effectively an empty operation. You are recommended to change your Gemfile as soon as is practical.

require 'mime/types/columnar'

Note that MIME::Type::Columnar and MIME::Types::Columnar are considered private variant implementations of MIME::Type and MIME::Types and the specific implementation should not be relied upon by consumers of the mime-types library. Instead, depend on the public implementations (MIME::Type and MIME::Types) only.

Cached Storage

mime-types supports a cache of MIME types using Marshal.dump. The cache is invalidated for each version of the mime-types-data gem so that data version 3.2015.1201 will not be reused with data version 3.2016.0101. If the environment variable RUBY_MIME_TYPES_CACHE is set to a cache file, mime-types will attempt to load the MIME type registry from the cache file. If it cannot, it will load the types normally and then saves the registry to the cache file.

The caching works with both full stores and columnar stores. Only the data that has been loaded prior to saving the cache will be stored.

mime-types Modified Semantic Versioning

The mime-types library has one version number, but this single version number tracks both API changes and registry data changes; this is not wholly compatible with all aspects of Semantic Versioning; removing a MIME type from the registry could be considered a breaking change under some interpretations of semantic versioning (as lookups for that particular type would no longer work by default).

mime-types uses a modified semantic versioning scheme. Given the version MAJOR.MINOR:

  1. If an incompatible API (code) change is made, the MAJOR version will be incremented, MINOR will be set to zero, and PATCH will be reset to the implied zero.

  2. If an API (code) feature is added that does not break compatibility, the MINOR version will be incremented and PATCH will be reset to the implied zero.

  3. If there is a bugfix to a feature added in the most recent MAJOR.MINOR release, the implied PATCH value will be incremented resulting in MAJOR.MINOR.PATCH.

In practical terms, there will be fewer releases of mime-types focussing on features because of the existence of the [mime-types-data][] gem, and if features are marked deprecated in the course of mime-types 3.x, they will not be removed until mime-types 4.x or possibly later.

Code of Conduct