[hypervis0r]

You don't see .jpeg because people are used to .jpg. There's nothing incompatible with long names that we care about.

Also, that Windows feature you mention is called ADS - alternate data stream.

[jandrese]

Yes, but you also never see extensions like:

.jpeg.photoshop.ro.creat127339292....

The filename extension is the bare minimum of metadata for a file and not easy to extend.

Even then Unix systems will skip even that minimal metadata and force you to messily search for magic numbers at the start of the file and make a guess.