I'm sure all but the most novice Good Booters are aware all files have a file extension that indicates the type of file it is and in most cases allows Windows to identify the program that created it.
As an example: A .doc file is a Word file extension added to the name you gave a Word document when you clicked "Save As." Because Windows knows a .doc file is a Word file, when you click on it Windows opens it in Word.
With some exceptions, most image files e.g., .jpg's can be opened by any graphic program and most music files e.g., .MP3 files can be opened by just about any media player.
The point is - Windows opens a file by identifying its file extension. However, if Windows cannot identify a file's extension it will not be able to open that file.
If you do not see your file extensions and you're a bit beyond novice and understand the importance of file extensions, you can change the way your files are presented.
Open your Control Panel and click on Folder Options. Click "View" and scroll down and un-check "Hide extension for known file types." Click Apply and OK. Your file extensions will now be visible.
If your file extensions are visible try this: Open your Pictures folder (My Pictures in XP) and "View" your photos as Large Icons (Thumbnails in XP). Right click a .jpg file, click rename and delete just the .jpg. Click "yes" when you receive the warning. Note that the Icon or Thumbnail is gone and if you click on the file you'll be asked to associate it with a program to open it. By deleting the .jpg file extension Window no longer knows what kind of a file it is and thus how to open it.
Note: When you right click on a Windows 7 file to rename it only the name of the file will be highlighted. To delete the .jpg, left click the highlighted area to remove the highlight from the file name and then delete the .jpg.
To restore the Icon or Thumbnail view of your photo, right click on the file and add the file extension .jpg.
This little exercise demonstrates why Windows doesn't by default display file extensions on many new computers. By hiding a file's extension and only presenting a file's name, when a novice changes a file's name they'll not accidentally delete or change the file's extension.
