diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index 8304d4dea..627c1a181 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -201,7 +201,9 @@ class Attachment < ActiveRecord::Base
   end
 
   def thumbnailable?
-    image? || (is_pdf? && Redmine::Thumbnail.gs_available?)
+    Redmine::Thumbnail.convert_available? && (
+      image? || (is_pdf? && Redmine::Thumbnail.gs_available?)
+    )
   end
 
   # Returns the full path the attachment thumbnail, or nil
diff --git a/test/unit/attachment_test.rb b/test/unit/attachment_test.rb
index fbbe71fbe..abe42587c 100644
--- a/test/unit/attachment_test.rb
+++ b/test/unit/attachment_test.rb
@@ -420,7 +420,12 @@ class AttachmentTest < ActiveSupport::TestCase
     assert_equal true, Attachment.new(:filename => 'test.jpg').thumbnailable?
   end
 
-  def test_thumbnailable_should_be_true_for_non_images
+  def test_thumbnailable_should_be_false_for_images_if_convert_is_unavailable
+    Redmine::Thumbnail.stubs(:convert_available?).returns(false)
+    assert_equal false, Attachment.new(:filename => 'test.jpg').thumbnailable?
+  end
+
+  def test_thumbnailable_should_be_false_for_non_images
     assert_equal false, Attachment.new(:filename => 'test.txt').thumbnailable?
   end