Some sort of machine-readable format for invoices and documents with related purposes (offers, bills of delivery, receipts,...) would be useful to get rid of some more of the informal paper or PDF human-readable documents we still use a lot. Ideally something JSON-based, perhaps with a cryptographic signature and encryption layer around it.
This one exists. SEPA or ISO20022. Encryption/signing isn't included in the format, it's managed on transfer layer, but that's pretty much the standard every business around here works and many don't even accept PDFs or other human-readable documents anymore if you want to get your money.
In Finland it kinda-sorta does, for some companies (mostly for things where you pay monthly). You can get your invoices directly to your banking account and even accept them automatically if you wish. And that doesn't include anything else than invoices, so not exactly what you're after. And I agree, that would be very nice.
Some companies, like one of our major grocery chain, offer to store your receipts on their online service, but I think that you can only get a copy of the receipt there and it's not machine readable.
It doesn't have any standardized extension. My solution uses .xml (as that's the format internally), but it's not anywhere in the standard. About category I don't really know. SEPA stands for Single Euro Payment Area, but it contains quite a lot of things, https://www.iso20022.org/about-iso-20022 has a bit more info on the standard itself, but there's no catchy category name either.