Index: Gemfile
===================================================================
--- Gemfile	(revision 17470)
+++ Gemfile	(working copy)
@@ -7,7 +7,7 @@
 gem "rails", "5.2.1"
 gem "coderay", "~> 1.1.1"
 gem "request_store", "1.0.5"
-gem "mini_mime", "~> 1.0"
+gem "mini_mime", "~> 1.0.1"
 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 17470)
+++ lib/redmine/mime_type.rb	(working copy)
@@ -64,7 +64,7 @@
       if extension.present?
         @known_types ||= Hash.new do |h, ext|
           type = EXTENSIONS[ext]
-          type ||= MiniMime.lookup_by_filename("a.#{ext}").try(:content_type)
+          type ||= MiniMime.lookup_by_extension(ext).try(:content_type)
           h[ext] = type
         end
         @known_types[extension]