[Lazarus] Function to get the mimetype from a file extension

[Alberto Narduzzi]

> the 'file' command looks at the magic numbers, but for example KDE looks at
> the extension.

my KDE, sic 3, looks at the magic numbers... I can rename a .pdf to 
whatever, even no extension at all, an it will still display the correct 
icon and open it with adobe reader.

So maybe with KDE 4 they did the same silly OS X move...

A.