How do I know which program is used to open a particular type of file?

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Summary: It's not always obvious what program should be used to open certain types of files. There are clues if you know where to look and what to look for.

Ever wonder why most filenames end in a period followed by three characters? Like ".exe" or ".doc"? That part of the file name, known as the extension, tells the operating system what kind of file it is and then from that what program should be used to operate on it. If the operating system doesn't already know, then you can use that same information to figure it out what program should be used.

Well, usually you can.

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When a program is installed in Microsoft Windows, part of the installation records the information about what file types that program should be used for. For example when Microsoft Word is installed, it tells Windows "I'm the program responsible for acting on '.doc' files." Thereafter when you double click on a ".doc" file, Windows knows to go fire up Microsoft Word to open the file.

One program can support multiple different file extensions. Microsoft Word, for example, will open ".doc", ".docx", ".rtf" and more. Most media players will play many different types of media files, and will not only register themselves for all of them, but they'll use the file extension to figure out what type of media file it is.

Unfortunately there are several problems with this system:

Multiple applications may try to register themselves as being responsible for the same filename extension. We see this all the time with media players: you download and install one program to play mp3 files, and it properly tells windows "I'm the program to play '.mp3' files". Later you find you need to install a different media player for some other reason unrelated to mp3 files - and yet it also tells Windows "I'm the program to play '.mp3' files". The last program to register that "wins", so the next time you try to play an mp3 file you don't get the program you expect.

Multiple applications may validly register themselves for the same filename extension. It's not a problem, per se, but it can be confusing. For example Microsoft Word will open ".doc" files, but so will OpenOffice Writer. And so will several other word processing programs.

The same file extension may be used for different and completely unrelated purposes. There's no official list of file extensions, and several are over used in incompatible ways. The most common is the ".dat" file extension which is used by many different program for many different things. There's no way to know which application to use if all you have is ".dat" to go on.

Anyone can use any extension for any purpose. Yes, ".doc" files are usually documents, and are usually handled by Microsoft Word, but there's no requirement that it be that way. I could dream up some new file type that has nothing to do with documents and which uses only my proprietary program. There's nothing to stop me from giving it the ".doc" extension and confusing a lot of people.

So, how do you know what program to use when you encounter a "no association" situation?

• Determine the filename extension; the (typically three) characters after the last period in the filename. For example in "resume.doc" it'd be the "doc".

• Determine what file type that extension might mean. If I don't already know, I turn to resources like Filext.com. Here you can enter the file extension and get a list of what types of file that might indicate.

• Yes, you'll get a list of possibilities and now you'll have to figure out which item on the list is the most likely to be appropriate for your situation. Even a search for ".doc" turned up 11 possibilities. The first was Microsoft Word, but there were 10 other options as well. Use what you know about the source of the document to pick from the list. Or ask the person who provided you with the file.

• Once you know what kind of file it is, you'll need to see if you already have software on your machine that will open it. If you haven't installed Microsoft Word, for example, you may not be able to open ".doc" files. If the software is installed but you're still getting the "no association" error message, then all you should need to do it make the association by hand.

• If you don't already have software that understand that particular file type, you'll need to find some and install it. Sometimes that means purchasing software to do so, but often there are free alternatives. For example as I mentioned Open Office, above. It's a free alternative that will operate on Microsoft Word documents. Alternately Microsoft makes available a Word Viewer which will allow you to view, but not modify, Word documents. Quite often there are similar alternatives for other types, but not always.

It can be confusing. Fortunately once things are set up properly for the file types you regularly deal with, it's not something you need deal with often.

Related:

• Ask Leo! - This file does not have a program associated with it - what does this mean and what do I do?

• Ask Leo! - PPS files still don't work even after I created an association - what do I do?

• Ask Leo! - What's a '.dat' file?

• Ask Leo! - Where did file associations go in Windows Vista?