Start > Programs > Windows NT 4.0 Option Pack > Microsoft Internet Information Server > Internet Service Manager

This will bring up the Microsoft Management Console (MMC) which is the new name (replacement) for Internet Service Manager.

Select Your Computer (If you are controlling more than one computer you need to set the MIME for each as required)

Console Root
    Internet Information Server
        Your Computer

This will set the MIME for this computer and all the Directories and Virtual Servers on it.

Right click on 'Your Computer' and Select Properties

Select the File Types button to configure the MIME (Multipurpose Internet Mail Extensions) mappings. These mappings set the various file types that the Web service returns to browsers. The registered file types that are installed on the server are listed in the File Types dialog box.

First check if there is a listing for .swf, if one is there edit it to the correct MIME type. If not there, click the New Type button in the File Types dialog box.

In the File Type dialog box enter .swf  in the Associated Extension box. In the Content Type (MIME) box enter application/x-shockwave-flash
Then press OK twice to close the open boxes and to Save the settings.

Other MIME settings you might want to add while there:

.spl    application/futuresplash
.mid    audio/midi