Zendesk Support provides the ability to restrict customers from adding attachments, or to enable all attachments:
However, this is a global setting and there's no granular control. Many businesses would find their security posture greatly enhanced by the ability to filter allowed attachments by type. For example, some businesses might find image and PDF attachments useful, but would need to block certain other document types and ZIP files.
Please consider empowering admins to exercise greater control over what email attachments can be added by end users to a Zendesk Ticket by allowing them to set allowable attachments by type. This is already possible with the Chat service: https://support.zendesk.com/hc/en-us/articles/360022183834-Managing-file-sending-options