So you tell your computer to open the file, and your computer says to you: "EXT??? What the heck is THAT!?" and refuses to open it. (Actually, what your computer tells you is "You do not have a program associated with this file extension"). Your computer has no idea what a file called file.ext is, or what program to use to open it. So you have to tell it which program to use to open files of this type.
Open up My Computer. Click on the pull-down menu VIEW and select Options (this may be called Folder Options). In Options click on the File Types Tab. In this screen click on the New Type button.
For "Description of type" type in: attachment For "Associated Extension" type in: ext (no period) Click on the down arrow at the end of the "Content type MIME" window and scroll down to choose: Text/Plain to go into that window.
Next click on the New button under the Action window. In this next screen, type: open in the window labeled "Action", then click on the browse button at the end of the window labeled "Application used to preform action". Browse to your C:/Windows folder, then look through this folder to find notepad.exe (or it may just be called Notepad) and select that and click on the Open button.
That will take you back to the "New Action" window. Click OK to close that window then click Close to close the next window.
If you know that your file is not a text file, then you really need to already know what kind of file this is (if it is a picture, is it a GIF? A JPG?) ahead of time. If it is a GIF graphic, then in the "Content Type MIME" window you would choose "Image/GIF" to go into that window, and in the "Application used to preform action" window you would need to browse to an application that can open GIF files. Explorer (explorer.exe or iexplore.exe) can open a GIF image file.