diff --git a/lib/plugins/acts_as_attachable/lib/acts_as_attachable.rb b/lib/plugins/acts_as_attachable/lib/acts_as_attachable.rb
index ae7e605..a816490 100644
--- a/lib/plugins/acts_as_attachable/lib/acts_as_attachable.rb
+++ b/lib/plugins/acts_as_attachable/lib/acts_as_attachable.rb
@@ -87,7 +87,7 @@ module Redmine
                 a = Attachment.find_by_token(token)
                 next unless a
                 a.filename = attachment['filename'] unless attachment['filename'].blank?
-                a.content_type = attachment['content_type']
+                a.content_type = attachment['content_type'] unless attachment['content_type'].blank?
               end
               next unless a
               a.description = attachment['description'].to_s.strip
diff --git a/test/unit/attachment_test.rb b/test/unit/attachment_test.rb
index 4799229..6a077fc 100644
--- a/test/unit/attachment_test.rb
+++ b/test/unit/attachment_test.rb
@@ -241,6 +241,15 @@ class AttachmentTest < ActiveSupport::TestCase
     end
   end
 
+  test "Attachment.attach_files should preserve the content_type of attachments added by token" do
+    @project = Project.find(1)
+    attachment = Attachment.create!(:file => uploaded_test_file("testfile.txt", ""), :author_id => 1, :created_on => 2.days.ago)
+    assert_equal 'text/plain', attachment.content_type
+    Attachment.attach_files(@project, { '1' => {'token' => attachment.token } })
+    attachment.reload
+    assert_equal 'text/plain', attachment.content_type
+  end
+
   def test_latest_attach
     set_fixtures_attachments_directory
     a1 = Attachment.find(16)