Excel detects that it is being asked to open a text-based file disguised as a binary spreadsheet and warns the user that the file could be corrupted or unsafe . How to Fix the Error 1. Use a Dedicated Excel Library
If you are fetching the file from a server using XMLHttpRequest or fetch , ensure you set the responseType to 'blob' . This ensures the browser handles the binary data correctly rather than trying to parse it as a string. 4. The "Save as HTML" Workaround
If you are an end-user receiving this file and trust the source:
The most reliable way to avoid this error is to generate a real .xlsx file (Office Open XML) instead of spoofing an HTML table. Libraries like SheetJS (xlsx) or ExcelJS create valid binary files that Excel recognizes immediately without warnings. 2. Change the Extension to .csv
In many JavaScript "export to Excel" implementations, developers wrap a standard HTML or CSV data in a Blob and force the browser to download it with a .xls extension.