I am unable to attach any document. Please let me know the reason.
The most common reason why you face this problem is that the size of the file that you are trying to open or download is too big.(i.e more than 10 MB)
Another factor which causes a problem is a poor Internet connection. If your attachments are fairly large and you have a poor Internet connection, to the Internet, then the attachments will take a very long time to get downloaded. This delay will result in a time-out and you will not be able to attach the file.