NWA-PCUG Newsletter Article, February 2005
Why Can't I Open This File?
by Brian K. Lewis, Ph.D.
Member of the Sarasota Personal Computer Users Group, Inc., Florida


There are times in every computer user's life when the computer seems to be deliberately trying to increase your level of frustration. One of these little moments is when you receive a file from an associate and no matter what you do, you can't open it. The file can be a document, a picture, a spreadsheet or any number of other types. So why does this happen? And what does it have to do with these things called extensions or suffixes?

Believe it or not, every file name on your computer has a three digit suffix. I know if you have never used any OS other than Windows you may never have seen this suffix or file extension. But they really do exist. If you open "My Computer" or Windows Explorer, select "Tools" from the menu. Then click on "Folder Options" and in that window click on "View". In this list some items are checked and some are not. If there is a check mark in the item "hide extensions for known file types", remove it. Then click on "apply to all folders". You will get a message telling you that the change will occur the next time you open a folder. Now open up any folder and look at the array of file extensions. Go from folder to folder and see how many different extensions you can find.

So now you see that the number of different extensions seems to be limitless. Or at least enough to bring on some confusion. Why is there such a proliferation of these three figure suffixes? Let's look again at the "Tools" menu and "Folder Options". This time click on the tab for "File Types". The upper window lists "Registered file types". Now scroll through the list until you find "Adobe Acrobat Plug-in file". This has an extension of API and in the lower window you find a short explanation as to which program can open this file. In this case, Adobe Acrobat. Keep scrolling down through the list. You will probably find the BMP suffix. This is a picture file and usually opens with Microsoft Paint. However, on my computer it has been associated with IrfanView, a graphics viewer. Keep scrolling to get an idea of all the different file types and their extensions.

If you click on enough different file types you will notice that each type is generally, but not always, associated with an application. When you click on the change button, the preferred application is listed at the top. You can change this to another application but then double-clicking a file of that type may result in a error and it won't open. Applications such as Microsoft Word, Word Perfect, Microsoft Works, Excel, etc., can only open files that were created by them unless they have the appropriate translator for the document. This goes back to the reasons for the file suffixes. The suffix tells Windows what application is needed to open a particular file. Every application capable of creating files uses a different format for the file header and body information. This formatting allows Word, for example, to open a document that has specific margins, type face, printer assignment, etc. The same is true for other applications.

Let's take a look at some of the definitions associated with the file structure of a Word file.

"FIB (File Information Block): The header of a Word file. Begins at offset 0 in the file. Gives the beginning offset and lengths of the document's text stream and subsidiary data structures within the file. Also stores other file status information." "DocFile: A Word docfile consists of a main stream, a summary information stream, a table stream, a data stream, and 0 or more object streams which contain private data for OLE 2.0 objects embedded within the Word document."

"Document: A named, multi-linked list of data structures, representing an ordered stream of text with properties that was produced by a user of Microsoft Word."

"Datastream: The stream within a Word docfile containing various data that hang off of characters in the main stream. For example, binary data describing in-line pictures and/or form fields."

From these descriptions you can see that a Word document has a very specific structure that has to be read and interpreted in a specific way. Other applications can't read a Word file unless they have a translator for it. Even then, the translation may not accurately reflect the content and format of the original file.

Another file format is that of database files. In a dBASE file, or a FoxPro file, the first byte identifies the version that created the file. Consequently, older versions can't read files created with newer versions of the program. For a number of years Microsoft created similar problems in Word documents by changing the file format every time a new version of Word was released.

Another application that has it's own file formats is Microsoft Works. This application can create text documents, database files and spreadsheets. None of these files can be opened by other programs. The interesting thing is that Works can save files in Word format, WordPerfect format or RTF (rich text file). It can even open most of these formats. But Word and WordPerfect are unable to read files in the native Works format.

Another problem exists with picture (graphic) files. Although your web browser should be able to read the most common formats, they have to be associated with the browser. If they are not, you will get an error when you attempt to open the file. Every graphic format has a different structure and may not always open in your photo software.

The most common formats used on the web are GIF and JPG (jpeg). However, these are two very different formats. GIF is a compressed format that is referred to as a "lossless" compression. In other words, you don't lose any detail in the process of compression/decompression. However, GIF can not use more than 256 colors. That makes it less usable for color photos than other formats. However, for web page logos and other small graphics, GIF is ideal. The file size is small, so transmission time over the Internet is quite short.

Photographs can be sent by e-mail using the JPG format which is very compressible. Jpegs can be compressed to 10% of their original size which greatly reduces transmission time. However, the greater the compression, the greater the loss of detail. Jpegs are a "lossy" format. The detail that is lost by compression can never be recovered. If the picture is important, you should always keep an uncompressed master in a safe location.

Another "lossless" graphic format is TIF or TIFF (tagged image file format). This is the best format for color pictures and should be used to save the master copy of important photographs. The major problem with TIF files is that they are very large. Much larger than jpegs. For example, a file from a digital camera was 526 KB in its native format. When converted to TIF and compressed, it took up over 6 MB on the hard drive. However, not every graphics program can read TIFF files. If yours can't, then you need something like IrfanView, a free file viewer.

So the answer to the original question is: you either don't have the application installed on your computer or you don't have the application associated with the file you are trying to open. Now, if you know the application that created the file and if you have it on your computer, you can solve the problem. You only need to associate the file with application. To do this open "My Computer" and select the "Tools" menu and "File Options". Click on the "File Types" tab and then scroll to the file extension for the file you want to open. Click on "Change" and select the application you want to use to open the file.

All of this discussion assumes that the file has not been corrupted. Even minor damage to the header of file can keep it from being opened. Transmission of files between computers is always subject to possible damage. Now maybe those undamaged files can be opened with a little less frustration.

Dr. Lewis is a former university & medical school professor. He has been working with personal computers for more than 30 years. He can be reached via e-mail at bwsail@yahoo.com.

There is no restriction against any non-profit group using this article as long as it is kept in context with proper credit given the author. The Editorial Committee of the Association of Personal Computer User Groups (APCUG), an international organization of which this group is a member, brings this article to you.

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