If the .009 file size is significantly different from parts .001 through .008 , it may be a partial download.
Ensure you have 5.7z.001 , 5.7z.002 ... up to the final number. Use 7-Zip: Right-click the first file ( 5.7z.001 ). Extract: Select "Extract Here" or "Extract to folder." 5.7z.009
If part .008 or .010 is missing, the extraction will fail with a "Payload error" or "Unexpected end of data." If the
[Archive Name].[Extension].[Part Number] 5.7z.009