Recently we were facing some issues while trying to import large Excel files to the system (Staging and Live environment). We would either get some invalid column warning (though the columns were correctly formatted) or the file would upload successfully but would skip a lot of rows. For example, when we were importing 19K+ rows, it would say ‘File uploaded successfully’, but we would only get some 15K rows in the database.

Turns out, for IIS 7.5, you will have to save the excel file as type ‘Excel 97-2003 Workbook’ for the successful import. Again, this type is necessary when we have a large excel file. For smaller files, it does not matter which workbook type you use.

Hopefully, this goes away, once we have IIS 8.0/8.5, but those versions are only available for Windows Server 2012/R2 and Windows 8/8.1.