diff --git a/actionpack/lib/action_controller/mime_type.rb b/actionpack/lib/action_controller/mime_type.rb
index fa123f7..baf3a4f 100644
--- a/actionpack/lib/action_controller/mime_type.rb
+++ b/actionpack/lib/action_controller/mime_type.rb
@@ -53,7 +53,7 @@ module Mime
       end
 
       def lookup_by_extension(extension)
-        EXTENSION_LOOKUP[extension]
+        EXTENSION_LOOKUP[extension.downcase]
       end
 
       # Registers an alias that's not used on mime type lookup, but can be referenced directly. Especially useful for
@@ -68,7 +68,7 @@ module Mime
         SET << Mime.const_get(symbol.to_s.upcase)
 
         ([string] + mime_type_synonyms).each { |string| LOOKUP[string] = SET.last } unless skip_lookup
-        ([symbol.to_s] + extension_synonyms).each { |ext| EXTENSION_LOOKUP[ext] = SET.last }
+        ([symbol.to_s] + extension_synonyms).each { |ext| EXTENSION_LOOKUP[ext.downcase] = SET.last }
       end
 
       def parse(accept_header)