Getting your computer to use the correct program when you open an email attachment is related to the type of file that attachment is, such as a picture or a music file. Once you change the default program for that file type, your computer will use the correct program from then on. You'll use almost the exact same method for this whether you use webmail, like Yahoo!, Gmail or Hotmail, or you use an email client, like Outlook or Thunderbird.
Go to your inbox in your web browser if you use webmail or in your email client on your computer.
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Click on the email with the attachment you want to view.
Download the attachment by right-clicking it and selecting the download option. Most browsers and email clients have options like "Save link as," "Save target as" or just "Save as." Save it wherever you want.
Locate the file you just downloaded on your computer and right-click it, then highlight "Open with;" in a moment a list of options will pop out to the side.
Select "Choose program" to open the "Open with" window, which contains a list of programs for you to choose from.
Check the box next to "Always use the selected program to open this kind of file."
Select the program you want to use for opening this kind of file. If you don't see the program, click the arrow next to "Other Programs" to display more.
Click "OK" to change the default setting and open the file you downloaded.
Delete the file you downloaded if you no longer want it.