Forum attachments not ordered in forum as displayed at upload time

Identified in MDL-41708; now that we're showing attachments in the forum by timemodified, we should also show attachments at upload time ordered by timemodified.

Repro steps:

1. Make sure you have the fix for MDL-41708 applied.
2. Open a forum.
3. Edit settings and ensure that you can upload multiple attachments.
4. Take a set of images and name them 1, 2, 3 etc.
5. Create a new forum post.
6. Upload attachment 3, then 2, then 1.
   • Observe that although you uploaded in reverse order - 3-2-1 - the attached items panel shows your images ordered by name.
7. Save and display your post.
   • Observe that your images are displayed in the forum in the order you specified, 3-2-1.