[scpedicini]

This is one of those ideas that seems nice in theory but in practice would just lead to so many subtle problems especially if it happened automatically.

Lots of programs create temporary files by changing the extension temporarily but if the kernel were attempting to intercept and change the internal data you could never guarantee the state of your file data. Not to mention potentially renaming an extension to an automatically handled extension and having that data irrevocably altered without you even realizing it.

Then of course there's the fact that there are usually lots of options associated with converting a file. Jpg needs visual compression levels, mp3 needs bitrates, etc.