From 9272e2f41f81126b32552ad63e0aac1b31da0b63 Mon Sep 17 00:00:00 2001
From: MAEDA Go <maeda@farend.jp>
Date: Thu, 20 Aug 2026 19:15:46 +0900
Subject: [PATCH 1/3] Extract the fallback for undetermined content types into
 Redmine::MimeType.infer_type

---
 app/controllers/attachments_controller.rb |  7 +------
 app/helpers/icons_helper.rb               |  8 +-------
 lib/redmine/mime_type.rb                  | 10 ++++++++++
 test/unit/lib/redmine/mime_type_test.rb   | 18 ++++++++++++++++++
 4 files changed, 30 insertions(+), 13 deletions(-)

diff --git a/app/controllers/attachments_controller.rb b/app/controllers/attachments_controller.rb
index 283803d7b..168b344ed 100644
--- a/app/controllers/attachments_controller.rb
+++ b/app/controllers/attachments_controller.rb
@@ -294,12 +294,7 @@ class AttachmentsController < ApplicationController
   end
 
   def detect_content_type(attachment, is_thumb = false)
-    content_type = attachment.content_type
-    if content_type.blank? || content_type == "application/octet-stream"
-      content_type =
-        Redmine::MimeType.of(attachment.filename).presence ||
-        "application/octet-stream"
-    end
+    content_type = Redmine::MimeType.infer_type(attachment.content_type, attachment.filename)
 
     if is_thumb && !content_type.start_with?("image/")
       # Thumbnails of non-image files are stored in PNG format
diff --git a/app/helpers/icons_helper.rb b/app/helpers/icons_helper.rb
index 0e1e03520..6027da0f7 100644
--- a/app/helpers/icons_helper.rb
+++ b/app/helpers/icons_helper.rb
@@ -160,13 +160,7 @@ module IconsHelper
   end
 
   def icon_for_mime_type(mime, filename = nil)
-    # Content type detection falls back to application/octet-stream when it
-    # cannot tell the type from the file contents. In that case only, guess
-    # the type from the filename, as AttachmentsController#detect_content_type
-    # does.
-    if filename.present? && (mime.blank? || mime == 'application/octet-stream')
-      mime = Redmine::MimeType.of(filename)
-    end
+    mime = Redmine::MimeType.infer_type(mime, filename)
 
     MIME_TYPE_ICONS[mime] ||
       MIME_TYPE_ICONS[mime.to_s.split('/').first] ||
diff --git a/lib/redmine/mime_type.rb b/lib/redmine/mime_type.rb
index e308d59e0..1b810338a 100644
--- a/lib/redmine/mime_type.rb
+++ b/lib/redmine/mime_type.rb
@@ -73,6 +73,16 @@ module Redmine
       end
     end
 
+    # Returns content_type, or the type guessed from name if content_type is
+    # undetermined, ie. blank or application/octet-stream
+    def self.infer_type(content_type, name)
+      if content_type.blank? || content_type == 'application/octet-stream'
+        of(name).presence || 'application/octet-stream'
+      else
+        content_type
+      end
+    end
+
     # Returns the css class associated to
     # the mime type of name
     def self.css_class_of(name)
diff --git a/test/unit/lib/redmine/mime_type_test.rb b/test/unit/lib/redmine/mime_type_test.rb
index 934fc037b..50a44d82f 100644
--- a/test/unit/lib/redmine/mime_type_test.rb
+++ b/test/unit/lib/redmine/mime_type_test.rb
@@ -37,6 +37,24 @@ class Redmine::MimeTypeTest < ActiveSupport::TestCase
     assert_nil Redmine::MimeType.of('test.unk')
   end
 
+  def test_infer_type
+    to_test = {
+      # A determined type is returned as is, even if the filename suggests
+      # another type
+      ['text/html', 'not-really-an-image.png'] => 'text/html',
+      # An undetermined type is resolved from the filename
+      ['application/octet-stream', 'test.pdf'] => 'application/pdf',
+      ['', 'test.pdf'] => 'application/pdf',
+      [nil, 'test.pdf'] => 'application/pdf',
+      # application/octet-stream is kept when the filename does not help
+      ['application/octet-stream', 'test.unk'] => 'application/octet-stream',
+      [nil, 'test.unk'] => 'application/octet-stream',
+    }
+    to_test.each do |args, expected|
+      assert_equal expected, Redmine::MimeType.infer_type(*args)
+    end
+  end
+
   def test_css_class_of
     to_test = {
       'test.txt' => 'text-plain',
-- 
2.55.0