The problem with declaring a file's type in the content itself is that you end up rendering all prior file formats obsolete because they lack this information.

The problem with declaring a file's type in the content itself is that you end up rendering all prior file formats obsolete because they lack this information. The better solution I've seen proposed is to store file type information as metadata in the filesystem itself, but this would lead to compatibility problems with archiving tools and any other scenario where a file is moved between differing file systems. File extensions are the best way to make determining a file type easy without parsing the whole thing while also maintaining cross and backwards compatibility.