[duskwuff]

With regard for the first two - that's good for zip.js, but the problem is that support for those features isn't universal. There's been a lot of fragmentation over the last 36 years (!).

As far as the last (file type detection) goes, the generally agreed upon standard is that file formats should be "sniffable" by looking for a signature in the file's header - ideally within the first few bytes of the file. Having to search through 64 KB of the file's end for a signature is a major departure from that pattern.