@Rocio Lechuga -
Welcome to the community. Currently in JSM cloud, when an agent attaching file to the case, he/she can only perform this task via issue commenting using the "paper clip" symbol.
If the file needs to be visible to the customer, then the agent will need to ensure the commenting option of "Reply to customer" are used. Only then the attachment will be available in the portal UI for the customer.
NOTE, there is an outstanding enhancement to bring in the Attachment liked button (similar to what one sees in Jira Software Application's issue view UI) for JSM issues - https://jira.atlassian.com/browse/JSDCLOUD-8576
Hope this helps.
Best, Joseph Chung Yin
Jira/JSM Functional Lead, Global Infrastructure Applications Team
Viasat Inc.
