Hi,
Question 1: is it possible that CA Service desk can receive and send base64 encoded/ decoded data for attachment.
The only way I see you can add attachment to SDM is by using CA SDM SOAP or REST API
Question 2: If suppose we are migrating incident ticket from CA Service Desk to Service Now what is the best way to send attachment.
You can create trigger that will fire on attachment of new file, and will execute spell code. The code will execute remote reference (your custom server side executable), that will get attachment by using REST API or uploadservlet and will send it to servicenow.
It will be quite challenging so good luck