How to enable the "Always use the selected program to open this kind of file" checkbox

Solution 1:

I found the answer at this now-closed (as off topic) Stack Overflow question: https://stackoverflow.com/q/2543254/253586

1. Launch regedit.exe
2. Navigate to the following subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts
3. Scroll down and find the file extension you are having problems with (for me it was .7z)
4. Expand it to view its sub keys
5. Find a sub key called "UserChoice" and delete it.

Solution 2:

Sometimes UserChoice isn't available using regedit.exe. According to an answer elsewhere, the following instructions will work for a given file extension:

1. Open the Start menu.
2. Type: default programs
3. Select Default Programs.
4. Click Associate a file type or protocol with a program.
5. Select the first line in the list that appears.
6. Scroll down or type the filename extension (e.g., .svg -- important to include the leading dot)
7. Click Change program button
8. Click the program you'd like to associate with that extension.
9. Click OK.
10. Click Close.

The desired program is now associated with the given file type.

Note: When choosing the program, notice that the option (Always use the selected program...) is still greyed out but, in this sequence of steps, the option is checked.