Please follow the below instructions to correct this problem
1) Go to Administration Tab > Attachments Library > Repositories
2) Select Repository you need to use and configure with SSL > Right-Click and View
3) Take note of the Server being used
4) Go to System > Servers > Click on the Server > Edit
5) Configure the "Attachment Servlet Path" to use correct hostname and port of SDM. For example: http://hostname_of_server:8080/CAisd/UploadServlet
6) Click the Save button
7) Uploading / Downloading the file again using SDM should work now.