Redmine

in thumbnail macro (lib/redmine/wiki-formatting/macros.rb),

          title = options[:title] || attachment.title

this line changes attachment.filename if attachment has non empty description attribute.

In Attachment::title method,

  def title
    title = filename.to_s
    if description.present?
      title << " (#{description})" 
    end
    title
  end

filename.to_s returns filename itself, so when attachment has descripton,
title << " (#{description})" changes not only title but filename.

I attached a fix patch for app/models/attachment.rb to deep copy from filename attribute to title attribute to keep filename attribute from modifing title attribute.
This patch is generate under svn r17391 (3.4-stable branch).

I have confirmed the issue. Here is a test to catch it.

Index: test/unit/attachment_test.rb
===================================================================
--- test/unit/attachment_test.rb    (revision 17424)
+++ test/unit/attachment_test.rb    (working copy)
@@ -258,6 +258,7 @@

     a = Attachment.new(:filename => "test.png", :description => "Cool image")
     assert_equal "test.png (Cool image)", a.title
+    assert_equal "test.png", a.filename
   end

   def test_new_attachment_should_be_editable_by_author

Failure:
AttachmentTest#test_title [test/unit/attachment_test.rb:261]:
Expected: "test.png" 
  Actual: "test.png (Cool image)" 

Committed to the trunk and stable branches. Thank you for your contribution.