Operating systems such as Mac OS X or Windows determine the format of a file based on its filename extension — the section of its name following the final period. For example, HTML documents are identified by names that end with .html (or .htm), and JPEG images by .jpg (or .jpeg).
Windows and Mac hide these file extensions by default. The creators of these systems were worried people might unwittingly change the extension and lose access to the file. Unfortunately, hidden file extensions open up your computer to security risks and increase the risk of accidentally deleting the wrong file.
For example, hiding the extension makes it possible to have what appears to be two or more identical filenames in the same folder. This is especially true when image files are needed in more than one format for different applications. For example, a company logo may be needed both in .tif format (for publishing) and .gif format (for web sites). With the extensions visible, these would appear as the unique filenames “CompanyLogo.tif” and “CompanyLogo.gif“. With the extensions hidden, these would both appear to have the identical filename “CompanyLogo“, making it more difficult to determine which to select for a particular application or which to delete when cleaning up a folder.
A further downside is that hiding such information can become a security risk. By having file extensions hidden, a malicious user can create an executable program with an innocent name such as “Holiday photo.jpg.exe“. In this case the “.exe” will be hidden and a user will see this file as “Holiday photo.jpg“, which appears to be a JPEG image, and unable to harm the machine save for bugs in the application used to view it. However, the operating system will still see the “.exe” extension and thus will run the program, which is then able to cause harm and presents a security issue. — from http://wapedia.mobi/en/File_type
To unmask your file extensions in Mac OS:
- Select Finder
- Select Preferences
- Click on Advanced cog icon
- Check box next to “Show all file extensions” and close window.
To unmask your file extensions in Windows XP:
Open any folder (for example “My Documents”). Next…
- Select Tools
- Select Folder Options
- Select the View Tab
- “Hide file extensions for known file types” entry should NOT be checked.
- Click on the Apply To All Folders button and close the window.
To unmask file extensions in Vista:
Open any folder (for example “Documents”). Next…
- Click on the Organize button
- Select Folder and Search Options
- Select the View Tab
- Scroll down the list to the Hide extensions for known file types item… make sure it is unchecked
For further information on identifying a file and discovering what it’s doing on your computer, read this excellent article from PC Today.