Hi @Jake-K 

first a screenshot to make clear how it is working behind the scenes:

In a Business Rule you could use the API GlideSysAttachment.writeContentStream() to have the full control over the file name (internal field name in the cat item) for the copied attachment. See https://developer.servicenow.com/dev.do#!/reference/api/vancouver/server_legacy/GlideSysAttachmentGl... for example

Hint: The copy() method in that API will not help you!

Maik