Windows 10 won't let me set default apps for file extensions

It is possible to add a new file association directly to the registry when the Windows interface does not work for new extensions:

• Start regedit and click on HKEY_CLASSES_ROOT
• Right-click on HKEY_CLASSES_ROOT and choose New and then Key.
• Name the key according to the file extension, e.g. .ext. (If exists already, skip this step.)
• Set the default value of the key you just created to some unused unique name, e.g. MyFileType. (If set already, skip this step.)
• Repeat the above to create another new key with the above name, e.g. MyFileType. (If exists already, skip this step.)
• Set the default value of this key to the name of the file type, e.g. File Type name.
• Right-click MyFileType and create a new subkey named DefaultIcon.
• Set the new key's default value to C:\path-to\executable,0 using the executable that is to handle the request. Pick another executable if yours does not contain icon resources. (0 is the index of the icon within the executable.)
• Create another new key and call it shell.
• For each verb/action you'd like to associate, create a new sub key with the desired name. Some verbs are preset and will automatically use localized names, e.g. open, edit or print.
• For each verb create a sub key command and set its default value to the program to be run. Also make sure to include all parameters needed, adding quotes if there are spaces inside, e.g. "C:\My Path\My Program.exe" "%1". %1 will be replaced with the actual file name picked for this action.

For instance, here's a sample registry file to create an association between .txt files and EmEditor:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.txt]
@="emeditor.txt"

[HKEY_CLASSES_ROOT\emeditor.txt]
@="Text Document"

[HKEY_CLASSES_ROOT\emeditor.txt\DefaultIcon]
@="%SystemRoot%\\SysWow64\\imageres.dll,-102"

[HKEY_CLASSES_ROOT\emeditor.txt\shell]

[HKEY_CLASSES_ROOT\emeditor.txt\shell\open]

[HKEY_CLASSES_ROOT\emeditor.txt\shell\open\command]
@="\"C:\\Program Files\\EmEditor\\EMEDITOR.EXE\" \"%1\""

[HKEY_CLASSES_ROOT\emeditor.txt\shell\print]

[HKEY_CLASSES_ROOT\emeditor.txt\shell\print\command]
@="\"C:\\Program Files\\EmEditor\\EMEDITOR.EXE\" /p \"%1\""

Windows 10 patch KB4467682

Addresses an issue that prevents some users from setting Win32 program defaults for certain app and file type combinations using the Open with… command or Settings > Apps > Default apps.

I can confirm that this recent patch fixes the issue. Just update Windows 10 and it should work again