Most of the server developers (ones that don't patiently wait for *something large*, though this resource is powerful enough) use their own server-side object attachments implementation that represents some large JSON with all the object and attachment information. This resource lets you cache "static attachments" in the client-side code so thanks to efficient utilization of the shared data it just uses a small shared variable to do the attachment processing without any need to create server-side objects nor send its data.
For example, if you want to attach a specific object to specific ped bone with spefic bone offset and object rotation, this will only use a ~4-8 byte shared variable keeping it accurate. User-made implementations I have seen before usually take ~100 bytes per an object.
mp.attachmentMngr.register(attachmentId, model, bone, offset, rotation)
There's an example resource in /client-packages/epic-attachments folder. It lets you toggle attachments (there are like 30 attachments) using a NativeUI-based menu. Thanks to root for his contributions made to the example resource.