File association broken under Windows

File association does not work under Windows.

The command in the registry in Computer\HKEY_CLASSES_ROOT\Applications\Company.MyApp.ext\shell\Open\Command is: 

C:/Program Files/Company/MyApp/myapp.exe "%1"

 instead of:

C:\Program Files\Company\MyApp\myapp.exe "%1"

The forward slashes are invalid, this makes it unable to run, so double-clicking on a file of type ".ext" generates an error and does not open the application.

My installscript.qs is the following:

function Component()
{
}
 
Component.prototype.createOperations = function()
{
    component.createOperations();
    if (systemInfo.productType === "windows")
    {
        component.addOperation("RegisterFileType","ext","@TargetDir@/myapp.exe \"%1\"","MyApp files","application/octet-stream",  "@TargetDir@/myapp.exe,1","ProgId=Company.MyApp.ext");
    }
}

 
Editing the registry to fix the path with backslashes seem to fix this issue.
I've seen that there exists a function installer.toNativeSeparators(), but I don't understand how to use it here.

I've tried with backslashese in the script, but only the last folder changes.