I have written code that reads the xlsx file out of Power Point 2007 pptx report. The routine works fine most of the time but there are Excel files that it tries to open that return an error that the file is not a valid Excel file. Obviously I have ensured that the Excel file is valid and I can open the Excel file externally using Excel 2007.
Any suggestions of what may cause these errors will be a massing help
Aspose.Cells supports to manipulate the native excel files including .xlsx format. Kindly make sure that those excel files are native Excel files (.xlsx files) and not originally html files or some other formats which are just Saved As .xlsx files (you may try to open the files into notepad and can check it source).
If this is not the case, could you post your template files here, we will check it soon.