diff --git a/Gemfile b/Gemfile
index afebe65b9..28472c17c 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,7 +4,6 @@ ruby '>= 3.2.0', '< 4.1.0'
 
 gem 'rails', '8.1.3.1'
 gem 'rouge', '~> 5.0'
-gem 'mini_mime', '~> 1.1.0'
 gem "actionpack-xml_parser"
 gem 'roadie-rails', '~> 3.4.0'
 gem 'marcel'
diff --git a/lib/redmine/mime_type.rb b/lib/redmine/mime_type.rb
index e308d59e0..733cefe95 100644
--- a/lib/redmine/mime_type.rb
+++ b/lib/redmine/mime_type.rb
@@ -46,8 +46,6 @@ module Redmine
       'image/png' => 'png',
       'image/tiff' => 'tiff,tif',
       'image/webp' => 'webp',
-      # Not registered with IANA, but matches Marcel's type for .ai
-      'application/illustrator' => 'ai',
       'application/javascript' => 'js',
       'application/pdf' => 'pdf',
       'video/mp4' => 'mp4',
@@ -66,10 +64,13 @@ module Redmine
 
     # returns mime type for name or nil if unknown
     def self.of(name)
-      ext = File.extname(name.to_s)[1..-1]
-      if ext
-        ext.downcase!
-        EXTENSIONS[ext] || MiniMime.lookup_by_extension(ext)&.content_type
+      ext = File.extname(name.to_s).delete_prefix('.').downcase
+      return if ext.empty?
+
+      EXTENSIONS.fetch(ext) do
+        type = Marcel::MimeType.for(extension: ext)
+        # Marcel falls back to application/octet-stream for unknown extensions
+        type unless type == Marcel::MimeType::BINARY
       end
     end
 
diff --git a/test/unit/lib/redmine/mime_type_test.rb b/test/unit/lib/redmine/mime_type_test.rb
index 934fc037b..a753f04bc 100644
--- a/test/unit/lib/redmine/mime_type_test.rb
+++ b/test/unit/lib/redmine/mime_type_test.rb
@@ -87,7 +87,7 @@ class Redmine::MimeTypeTest < ActiveSupport::TestCase
     assert_includes image_types, 'image/webp'
   end
 
-  def test_should_default_to_mime_type_gem
+  def test_should_fall_back_to_marcel
     assert !Redmine::MimeType::EXTENSIONS.key?("zip")
     assert_equal "application/zip", Redmine::MimeType.of("file.zip")
   end