2 Answers

File size: Larger files take longer to upload. Also, you can only add attachments up to your 25MB message limit. If you have an enormous file that won't fit within this limit, we apologize for the inconvenience!
Other programs running: If you have many other programs currently running, your available memory might be low. This could slow down the attaching process.  Recommend closing any programs you're not using, and then going through the attachment process again.
Slow connection speeds or browser issues: In this case recommend that you try using a different browser (if you are using Internet Explorer, try switching to Firefox, etc.).

Try rebooting your computer. Sounds lie a simple script error that a reboot should take care of.