I'm working on a web application and we have an administrative feature that allows admins to import an excel spreadsheet containing information about sub-users they would like to add. We parse the spreadsheet and do basic input validation on most fields. We also ensure that the file has the correct extension before starting doing any deeper examination of the file.
I'm just curious if there are any other risks I'm overlooking when dealing with imported user data. Specifically importing Excel Spreadsheets.