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How do I find the place to change to 'all" on my pc, is it in control panel?
No, that's a composite screenshot of the Import media Files... window, and you would click on the drop-down arrow for the Files of type: list. You'll need to do that each time you want to import one or more m4v files so that PD will not exclude them.
The bottom section of the screenshot shows what your m4v clips looks like whenPD is ready to import it.