I am not sure if it has been answered but I found that it is possible to do this using LaunchFileAsync.
An example is as follows:
//To launch app
var launchFileName = "june.kit";
var folder = Windows.Storage.ApplicationData.Current.LocalFolder;
var option = Windows.Storage.CreationCollisionOption.ReplaceExisting;
// create file
var launchFile = await folder.CreateFileAsync(launchFileName, option);
// write content
await Windows.Storage.FileIO.WriteTextAsync(launchFile, tempChk);
// launch file
bool success = await Launcher.LaunchFileAsync(launchFile);
Where launchFileName is possible to have any name with any extension, and this extension should mention in the calling application's FileTypeAssociation in the packager.manifest file.