Conflicted on filename extensions. For the average person it works just fine, and I suppose that's what probably matters. It's not very common for not knowing the details of how they work to matter. It's just silly that the same information is also in the start of the file 99% of the time. It is nice though to have a readable, usually reliable label, and then have a signature anyways for when different names overlap. Wikipeda lists 4 completely unrelated types with a .mod extension, for example.

Pretty much any application will correctly open any file type it supports, regardless of the extension. So it is quite unintuitive that you could have a file named ".png" that seems to work completely fine yet is actually a jpeg or something. But that hopefully isn't a case that people run into very often, so it probably doesn't matter.