The following reflects a subset of process steps from the Process Definition Document (PDD):
1. Read the unread emails from the Robot mailbox and mark them as read.
2. If the email has an attachment of type PNG, download the attachment.
3. Else, send a reply to the sender with the message "Invalid Attachment, please attach a .PNG file to process".
If you are designing a process based on the PDD steps, what is the correct exception type for Step 3?