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stream.Position = 0;
IWorkbook workbook = application.Workbooks.Open(stream);
I downloaded the file and was able to see the screen shot that you have attached. The corruption of the file is due to invalid extension. This happens very often in Excel. Here are the steps which is mostly used to fix it:
Method 1 – Change the Default File Format to Save Excel Workbooks
You can also read this thread: https://social.technet.microsoft.com/Forums/office/en-US/63020ccc-51d7-46d9-b956-121c0e6efcc8/excel-file-error-the-file-format-and-extension-dont-match?forum=Office2016ITPro