Received information from Cisco TAC:
"For version 10.6 the maximum attachment size is 2 MB. Any attempt to upload an attachment file with size > 2 MB is rejected. The size of a single attachment cannot exceed 2 MB. The total size of all agent attachments can not exceed 5 MB. A warning message is displayed and the last attached file is removed (because adding this file exceeded the limit).
The total number of attachments can not exceed 10. A warning message is displayed when the agent attempts to add the 11th file."
This limit has been increased from 2MB to 10MB in 11.6 for single attachment.