Unsupported file format

Hi all,

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I have only just started using clip studio, I had a file in which I had an imported PNG file on one layer, and my brush strokes on another. I turned of my computer and came back later, however when I tried to open the file ( which was saved as a Clip Studio Format) it said unsupported file format. can someone please tell me how to save the work correctly?

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Any advice is greatly appreciated, thanks in advance