Unknown File Extension or File Extension no longer recognized?

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Why Windows Stops Recognizing File Types

If Windows recognized a file extension yesterday, but not today, that means that the file association has been lost. Windows refers to an index that lists known file extensions along with their associated programs. For example, in this index, you would find PDF as a known file extension along with Adobe Reader as its associated program. When Windows encounters a .pdf file, it checks this index and sees that it should open Adobe Reader. When all is well, you click the PDF file and Reader launches. When the file association is missing, Windows doesn't know what program to use to open the file.

Resetting File Associations

For the most part, it's easy to reset a file association on your own. Of course, you need to know which program is used for opening the file so that you can tell Windows to use it.

Once you know which program is used, open Windows Explorer

Right-click Start
Choose Explore
Navigate to the file that you want to open (in our example, a .pdf file)
Right-click the file and choose Open With

Windows will recommend programs that it thinks might work

If Adobe Reader is listed, you'll want to highlight it
If it is not listed, use the Browse feature to search for it
Once selected, look for the box that says, "Always use the selected program to open this kind of file" and place a check mark in the box.
In our example, this tells Windows to always use Adobe Reader when a PDF file is clicked.

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