Opening a file for reading purpose is fruitful only if you open that with a right program. But this will not happen most of the times. The operating system you use will guess the program to open the file when you open known file extensions. You will be asked for the program to open with when you try to open an unknown file extension. When you try to open that file for the first time, you have to browse the list of programs and select the appropriate program.
Many people make a wrong guess and choose the wrong program to open the file. When you marked the tick on the checkbox ‘Always use the selected program to open this kind of file’, then similar files will also be opened with the wrong program. To fix this, you can right click on the file and select open with or choose program option depending on the operating system. Now select the correct program and press ok. This will open the file with the appropriate program.
If this option is not visible, you can use the folder options from the tools menu in the windows explorer. Go to file types tab and select the file extension. Then click on change in the details information area to change the default program and press apply.