So when I dumped these mysterious little units into Winamp I didn’t think they’d play. But they did.”

Congratulations, you’ve just discovered that file extensions have absolutely no effect on the contents of a file.

Back in the early days of home computers (AKA before the dawn of recorded history for most of today’s users), computers didn’t have a GUI. There were no mice and no icons to click on. When you wanted to run a program, you typed in its name. Once it was loaded, if you wanted to open other files with it, such as pictures, you used the “Open File” option provided by the program and usually selected the file to be opened by using the arrow keys to move the selection bar to the file you wanted. Although there were some programs that insisted on files having the proper extension, for the most part, filename extensions existed to serve as a visual reminder of what each file was.

When GUI based OSs came along, the idea of double-clicking to open files was born. You could run programs by double-clicking them and you could open data files by double-clicking them. To do this, the OS looks at the extension, looks up that extension in its internal list, sees what program is “associated” with that extension, loads that program and passes the file to it. The OS doesn’t really know what the file is, it just relies on the extension. If the extension is wrong, the file will be passed to the wrong program. Since this can lead to crashes when a program tries to process random data, file “headers” were invented. A file’s header contains information that tells the program what type of file it is. If the program doesn’t recognize the header, it will probably refuse to open/play/display the file. In the case of audio/video files, the header contains information about the codecs needed to play the file.

If you use the same program to view JPG and GIF files, you can rename one to the other and they will still work, because both extensions are passed to the same program. On the other hand, if you rename a JPG file to TXT, Notepad or whatever program you use for a text editor isn’t going to know what to do with the file.

This also means that removing the extension in no way changes what the file is, it only eliminates the OS’s ability to automatically pass it to the apropriate program when you double-click it. If you were to rename a file from “Some-Song.mp3″ to “Some-Song”, you haven’t changed anything other than the filename. It’s still an MP3 file. If you drag & drop that file on your media player, or you use the Open option from the file menu, it’s going to look at the file’s header, see that’s an MP3 file and play it.

There’s nothing magical about file extensions, they’re simply a convenience feature that allows you to load files by double-clicking and which serves as an easy indicator or what the files are.

“Files have no extension for a reason – to prevent users from wandering in the cache and opening files by double-clicking. Files in the cache can have malicious content and should not be opened just by double-clicking.

There is a MIME type served by server and there is a file association that is used to choose an application to open the file with. When the two do not match, we have e-mail viruses (e.g. file.exe marked as image/png, and Outlook Express running it as executable on preview).”

Which is a perfect example of why people shouldn’t be using any of the software that Microsoft provides with Windows, not even the picture and fax viewer. Any piece of software that is broken enough to execute an EXE labelled as JPG, should be disabled and/or uninstalled immediately and replaced with something designed by someone who knew what they were doing.

I just renamed a harmless EXE file (a small game) to several popular extensions;

JPG – Irfanview opened and displayed the program’s icon.

RAR/ZIP – WinRAR & WinZip both told me it was a bad archive.

AVI – Media Player Classic told me that the video was in an unknown format.

HTML – Firefox showed me a raw listing of the file’s contents.

PDF – Foxit Reader failed to open at all.

TXT – Metapad told me that it wasn’t a text file, asked if I wanted to load it anyway and then displayed the raw file contents.

TORRENT – uTorrent told me it was an invalid file.

DOC – My Word viewer (I don’t have Word) told me it was a bad file.

PAR2 – Quickpar opened, but ignored the file.

SFV – Fsum opened then immediately closed.

I’m failing to see the danger here, especially considering that 99% of today’s users have no idea what a browser cache is, let alone how to actually find it on their system. Anyone with the expertise to find the files would hopefully be intelligent enough to be using software that doesn’t act like a universal file opening system that tries to figure out what each file is and open it regardless.

And I can’t help commenting on the sad state of today’s computer competency level, when the ability to find files on your hard drive needs to be classified as “expertise” rather than a basic skill that every computer user should have.

There is a MIME type served by server and there is a file association that is used to choose an application to open the file with. When the two do not match, we have e-mail viruses (e.g. file.exe marked as image/png, and Outlook Express running it as executable on preview).

Unfortunately, there is still “magic” guessing in MS file handling, e.g. try to rename your file.doc to just “file” with no extension and double-click it.