*. What?

Baffled by the huge number of file extensions out there? Wish you knew what they meant? Even here at Windows we sometimes come across a file extension that has us totally perplexed, so we've decided to delve deeper expose the maddening mystery behind those cryptic little letters. Let's start with the basics…

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By  Matthew Wade Published  November 1, 2005

|~||~||~|An extension is a particular code at the end of a file name, which indicates the type of file (file format). Most extensions are three characters in length, however some employ two or even four characters. For instance, 'howitworks.xls' includes the file name 'howitworks' and the file extension .xls refers to a Microsoft Excel file. Know your files It's extremely important to know exactly what a file extension means before you open a file, as some virus writers add false file extensions to a file's name to trick users into thinking it's a harmless file. For example, a virus in the form of an executable - such as 'cascade.exe' - can be altered to simply show up on-screen as ‘cascade.doc.exe'. If you fail to enable full file extension visibility and someone sends you this file, you will only see 'cascade.doc', possibly leading you to believe that it is a harmless Microsoft Word document. Windows XP often hides file extensions by default and while this may ‘seem’ to simplify matters, it’s a potential recipe for trouble. To make file extensions visible on your machine, go to My Computer, choose Folder Options and select the View tab. Now uncheck 'Hide extensions for known file types' and click OK. You’ll now be able to see the extensions of all files in Windows Explorer. Although enabling file extension visibility can help you to spot many potential problem files, some high-level virus writers may change the original extension to something less obvious. For example, a virus writer may change the file brick32.exe to brick32.txt by simply renaming it. To this end, we also recommend that you refrain from opening files that originated as e-mail attachments from unknown senders. Also, make you sure you set-up your anti-virus software to scan all received files well before you actually get a chance to open them. Extensions exposed To make your life a little easier, we picked our communal brains to bring you details on the 16 most frustrating (in other words, often unknown) extensions around. We've also included a mini description of each extension, and also answered the ever-looming question, “What application can I use to open this pesky file?” Please note however, that all the applications we've listed may not open every single file with that particular extension. For instance, many programs often use .dat and .log files, however these can be in a format specific to the program that created the file. So, if one of our suggestions does not work for a particular file, it's important that you determine what program created the file first, and then try using that program to open it. At the same time, it’s also impossible for us to list each and every program that is capable of opening up files with a particular extension. To possibly determine which app to use for a particular file extension, we suggest you take clues from the location of the file as an indicator of which app is creating the file. Details of the file's date and time can also help if you can remember which app you were running when the file was created. If the file extension you'd like to know more about is not in this listing, here's a list of four comprehensive up-to-date online databases: - www.whatis.com - www.file-ext.com - www.cybertechhelp.com - www.computerhope.com ------------------------------------ *.arj Compressed file archive Details: Similar to .rar and .zip files, .arj files are archive files that store compressed data. These can be set-up to execute files immediately after decompression takes place, so it’s worth performing a virus scan of their contents before execution. Opens with: Zip Genius 5, ZipMagic, PicoZip, Stuffit Deluxe 9.0 *.bat Batch file Details: This extension is an MS-DOS batch processing file, which contains a number of line commands similar to that of a shell script. This file type is susceptible to infection and should be scanned very carefully before opening. Opens with: NotePad, WinBat, UltraEdit-32 Professional Text *.bin Binary file Details: These files are used by CD and DVD imaging software to store disc contents. They can then be used by the software to create duplicate copies of the original discs. Certain emulation software apps can also read the contents of .bin files to create virtual discs. Opens with: Nero Burning ROM, Alcohol 120% *.cab Cabinet file Details: This extension refers to a cabinet file that is commonly used by Microsoft to store installation files. Remember that this is a compressed file and shouldn't be tampered with lightly as data corruption may occur. Opens with: PicoZip, Zip Genius, ZipZag *.dll Dynamic Link Library Details: This is a support file used by all Windows-based programs. If several parts of a program need to perform the same action, that specific action may be placed into a .dll file that some of the program parts can all use. We suggest that you do not move or alter this kind of file. Opens with: Not applicable *.inf Information file Details: Besides standard executable files, many Windows components and other small applications can be installed using .inf files. These files can be edited by basic text editors and read to determine exactly what goes on during the installation process. Opens with: NotePad, Word Pad, Microsoft Word *.ini Initialisation file Details: This is a program initialisation file normally used by software that operates under Microsoft operating systems. .ini files contain software settings and environment variables, which programs may need to function properly. You can tweak an .ini, but always keep a back-up. Opens with: NotePad, Text Pad *.log Log file Details: .log files usually store complete details of programs that have successfully, or failed, to run. A .log file usually contains ASCII text. If a .log file is being used by the system or an app, you may not be allowed to open the file unless the utility you use opens it in read-only form. Opens with: UltraEdit-32 Professional Text *.pif Program Information File Details: A Program Information file is used to store data about DOS programs that run under Windows. Such files can be coded to execute commands in a linear or non-linear fashion. This is a potentially dangerous file type and you should never click on a .pif file without extensive knowledge. Opens with: Not applicable *.pst Personal Folders File Details: A Microsoft Outlook personal folder file. This includes all a user's e-mail addresses, e-mail messages, as well as attachments associated with Outlook or Outlook Express. If you plan to migrate to a new PC, make sure you back-up all .pst files so that you can import and use them on your new PC. Opens with: Microsoft Outlook, Outlook Express *.reg Registry Details: This is a standard Windows registry data file. These are self-executing and in most cases, if you open and alter them they will add their contents to the Windows Registry automatically. However, files of this type should never be altered unless you're 100% certain about what you're doing. Opens with: Windows Registry Editor *rtf Rich Text Format Details: This extension stands for rich text format and is a simple text file. This file is also sometimes used as Windows help file script. Unlike .doc files, .rtf files are generally much smaller in size as formatting and structure is stored using an ASCII character set. Opens with: Microsoft Word, Word Pad, NotePad *.sys System Details: These are typically MS-DOS system files, which set-up the operating environment in which software runs. Only edit these if you know exactly what you're doing. Also note that .sys files should not be deleted from the system unless you are absolutely certain they’re not in use. Opens with: NotePad or any text editor *.tmp Temporary Details: A temporary file can be used for a number of different purposes. Some programs create .tmp files to temporarily store data, and then automatically delete this when its purpose is served. We suggest you refrain from altering or deleting .tmp files as some apps hide critical data in these. Opens with: Not applicable *.txt Text Details: .txt files consist of plain ASCII code text. As a result, formatting such as bold, italic, underline, colours and graphics aren’t supported. Files of this sort can be opened and edited by any standard DOS or Windows-based text editor. Opens with: Microsoft Word, Word Pad, Note Pad, Jarte Word Processor *.3gp 3rd generation project Details: 3GP files are highly-compressed and contain multimedia data. These files are traditionally small in order to make sharing via high-speed wireless networks far quicker. Quality is generally compromised owing to their high compression. Opens with: Apple QuickTime 6.5 ,Real Player, Windows Media Player ||**||

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