Currently, if an attachment is accidentally deleted in Jira Cloud, there is no way to restore it apart from importing a backup. There should be a trash / soft delete / recycle bin from where these attachments can be retrieved.

Workaround

We recommend our users perform regular backups of their instance:

1. Choose the cog icon > Jira settings > System > Backup manager
2. Under Backup for cloud, select Create a backup for cloud

If you would like to know more about how to create a backup for your Jira Cloud instance, we have public documentation about it:
Export data from Jira Cloud

By having a backup, you could restore items from it.
How to import only part of a Jira export into Jira Cloud