IIS 6.0 404 Error for a file you *know* is there

This morning I was doing some maintenance on a customer's machine and needed to reinstall the application. The customer site is closed, but the work still needs to be done. I had a copy of the ISO media available on a server in our office, so the solution seemed obvious: drop it on a web server and pull it down from the client's machine.

So, I created a directory on one of our web servers, placed the ISO file there, and turned on "Directory Browsing" for that directory. I then logged into my customer's machine and navigated to the site and found the file. I right-clicked on the file, hit "Save Target As..." and ....

Internet Explorer cannot download myfile.iso from my.webserver.com.
Internet Explorer was not able to open this Internet site. The requested site is either unavailable or cannot be found. Please try again later.

I went back to the site and this time, just clicked on the file name, and was greeted with the generic 404 message.

Fortunately, the solution is very simple and only takes a few seconds.

1. Open up IIS Manager.
2. Navigate to the web site (if you don't want to change the setting for the entire web server, you can just navigate to the directory path off the web site).
3. Right-click > Properties on the website (or directory).
4. Select the "HTTP Headers" tab.
5. Select the "MIME Types" button.
6. Click the "New" button.
7. Add the extension type (in my case, ISO).
8. Add the MIME type (in my case, application/octet-stream).
9. Click OK three times.

Try to re-download the file in your browser.

The reason for this behavior is described in detail in the IIS 6.0 documentation here.