well I don't think we're on the same page here, in terms of what we're talking about. when you CLICK TO OPEN a file of any kind in the windows explorer (the window you have opened with code), the code has absolutely nothing to do with it. once the window is opened, the windows OS is in control (as far as I know anyway). file extensions are associated with programs by default. this might be an issue with the KIND of code you are using. I've never used what you are using:
Application.Dialogs(xlDialogOpen).Show
I seriously doubt that's it. but, if it is, try this instead:
Application.FileDialog (msoFileDialogFilePicker)
it also could be an issue with your regional setting, but I do not know. I live in the USA, so that would be unknown to me. but I doubt that's the issue either.