mime-types

https://github.com/mime-types/ruby-mime-types

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][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.