The problem is the case where you want to upload or attach >1 document that's actually a directory. You need a way to signal that the first 3 files are a part o

The problem is the case where you want to upload or attach >1 document that's actually a directory. You need a way to signal that the first 3 files are a part of document A, and the next 5 are part of document B, and although you could invent a file name convention to express this nothing understands it. Email clients would show 7 attachments, web server APIs would show 7 files, browsers would need to be patched to let you select bundles in the file picker and then recursively upload them, how progress tracking works would need to change, etc. And then how do you _download_ them? Browsers don't understand MIME at download time. None of it is hard to solve. But, nobody ever did, and the value of doing things this new way is usually going to be lower than the value of smooth interop with everyone's different browser/OS/email/server combos.