Index: Gemfile
===================================================================
--- Gemfile	(revision 17464)
+++ Gemfile	(working copy)
@@ -7,7 +7,7 @@
 gem "rails", "5.2.1"
 gem "coderay", "~> 1.1.1"
 gem "request_store", "1.0.5"
-gem "mime-types", "~> 3.0"
+gem "mini_mime"
 gem "actionpack-xml_parser"
 gem "roadie-rails", "~> 1.3.0"
 gem "roadie", "~> 3.2.1"
Index: lib/redmine/mime_type.rb
===================================================================
--- lib/redmine/mime_type.rb	(revision 17462)
+++ lib/redmine/mime_type.rb	(working copy)
@@ -62,14 +62,14 @@
     # returns mime type for name or nil if unknown
     def self.of(name)
       return nil unless name.present?
-      if m = name.to_s.match(/(^|\.)([^\.]+)$/)
-        extension = m[2].downcase
+      extension = File.extname(name)[1..-1]
+      if extension
         @known_types ||= Hash.new do |h, ext|
           type = EXTENSIONS[ext]
-          type ||= MIME::Types.type_for(ext).first.to_s.presence
+          type ||= MiniMime.lookup_by_filename("base.#{ext}").try(:content_type)
           h[ext] = type
         end
-        @known_types[extension]
+        @known_types[extension.downcase]
       end
     end