In addition, you validate the existence of an attachment by referencing the new function alias pxIsAttachmentOfCategoryInCase in a validate record. For example, you can use the properties and a validate record for building when logic that makes it necessary for a user to attach a document of category "SECCompliance" before the case can enter a stage.
A part from website is there any other way in your functionality the attachments gets attached to case. Generally there wont be any OOTB property stating whether the attachment has been submitted as part of website or through E-Mail or through any other channel. It should be set explicitly on the attachment page for every attachment based on the point from where the attachment is attached to the case.