diff -r 789932355683 app/helpers/application_helper.rb
--- a/app/helpers/application_helper.rb	Wed Jan 16 07:20:49 2019 +0000
+++ b/app/helpers/application_helper.rb	Wed Jan 16 15:40:26 2019 +0300
@@ -724,7 +724,7 @@
     attachments += obj.attachments if obj.respond_to?(:attachments)
     if attachments.present?
       text.gsub!(/src="([^\/"]+\.(bmp|gif|jpg|jpe|jpeg|png))"(\s+alt="([^"]*)")?/i) do |m|
-        filename, ext, alt, alttext = $1.downcase, $2, $3, $4
+        filename, ext, alt, alttext = $1, $2, $3, $4
         # search for the picture in attachments
         if found = Attachment.latest_attach(attachments, CGI.unescape(filename))
           image_url = download_named_attachment_url(found, found.filename, :only_path => only_path)
diff -r 789932355683 test/fixtures/attachments.yml
--- a/test/fixtures/attachments.yml	Wed Jan 16 07:20:49 2019 +0000
+++ b/test/fixtures/attachments.yml	Wed Jan 16 15:40:26 2019 +0300
@@ -268,3 +268,17 @@
   filename: root_attachment.txt
   filesize: 54
   author_id: 2
+attachments_021:
+  content_type: image/png
+  downloads: 0
+  created_on: 2011-02-23 16:14:50 +09:00
+  disk_filename: 101223161450_testfile_2.png
+  disk_directory: "2010/12"
+  container_id: 14
+  digest: 6bc2963e8d7ea0d3e68d12d1fba3d6ca
+  id: 21
+  container_type: Issue
+  description: ""
+  filename: TestТест.png
+  filesize: 3582
+  author_id: 2
diff -r 789932355683 test/unit/attachment_test.rb
--- a/test/unit/attachment_test.rb	Wed Jan 16 07:20:49 2019 +0000
+++ b/test/unit/attachment_test.rb	Wed Jan 16 15:40:26 2019 +0300
@@ -410,6 +410,30 @@
     set_tmp_attachments_directory
   end
 
+  def test_latest_should_get_with_unicode_downcase
+    set_fixtures_attachments_directory
+
+    # first unicode letter (\xd0\xa2) is uppercase
+    string = "Test\xd0\xa2\xd0\xb5\xd1\x81\xd1\x82.png".force_encoding("UTF-8")
+    assert_equal true, string.valid_encoding?
+
+    a1 = Attachment.find(21)
+    assert_equal string, a1.filename
+
+    # ensure that string#casecmp fails to compare unicode strings case-sensitive
+    assert_not_equal 0, string.casecmp(string.downcase)
+
+    # search with exact string succeeds
+    la1 = Attachment.latest_attach([a1], string)
+    assert_not_nil la1
+
+    # using downcase should fail because of casecmp
+    la2 = Attachment.latest_attach([a1], string.downcase)
+    assert_nil la2
+
+    set_tmp_attachments_directory
+  end
+
   def test_latest_attach_should_not_error_with_string_with_invalid_encoding
     string = "width:50\xFE-Image.jpg".force_encoding('UTF-8')
     assert_equal false, string.valid_encoding?
diff -r 789932355683 test/unit/helpers/application_helper_test.rb
--- a/test/unit/helpers/application_helper_test.rb	Wed Jan 16 07:20:49 2019 +0000
+++ b/test/unit/helpers/application_helper_test.rb	Wed Jan 16 15:40:26 2019 +0300
@@ -160,6 +160,16 @@
     end
   end
 
+  def test_attached_images_with_textile_and_uppercase_non_ascii_filename
+    # first unicode letter (\xd0\xa2) is uppercase
+    string = "Test\xd0\xa2\xd0\xb5\xd1\x81\xd1\x82.png".force_encoding("UTF-8")
+    attachment = Attachment.generate!(:filename => string)
+    with_settings :text_formatting => 'textile' do
+      assert_include %(<img src="/attachments/download/#{attachment.id}/Test%D0%A2%D0%B5%D1%81%D1%82.png" alt="" />),
+        textilizable("!" + string + "!)", :attachments => [attachment])
+    end
+  end
+
   def test_attached_images_with_markdown_and_non_ascii_filename
     skip unless Object.const_defined?(:Redcarpet)
 
@@ -170,6 +180,18 @@
     end
   end
 
+  def test_attached_images_with_markdown_and_uppercase_non_ascii_filename
+    skip unless Object.const_defined?(:Redcarpet)
+
+    # first unicode letter (\xd0\xa2) is uppercase
+    string = "Test\xd0\xa2\xd0\xb5\xd1\x81\xd1\x82.png".force_encoding("UTF-8")
+    attachment = Attachment.generate!(:filename => string)
+    with_settings :text_formatting => 'markdown' do
+      assert_include %(<img src="/attachments/download/#{attachment.id}/Test%D0%A2%D0%B5%D1%81%D1%82.png" alt="" />),
+        textilizable("![](" + string + ")", :attachments => [attachment])
+    end
+  end
+
   def test_attached_images_with_hires_naming
     attachment = Attachment.generate!(:filename => 'image@2x.png')
       assert_equal %(<p><img src="/attachments/download/#{attachment.id}/image@2x.png" srcset="/attachments/download/#{attachment.id}/image@2x.png 2x" alt="" /></p>),