Can I add more fields to the Attachment object in Application Composer?

This topic covers that you can't add more fields to the Attachment object but you can use the profile options to enable a few fields.

No. The attachment object isn't extendable but you can try using the profile options to enable some fields for entering more information. For example, you can set ZCA_ATTACHMENT_ENABLE_BASIC to No to enable Title and Description fields for the attachments object.