Question or Issue
When you export to excel in lightning, you receive the following error: The file format and extension of 'download.xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want ot open it anyway?
Causes or Possible Causes
This is a Microsoft error that is caused as a result of the method used to export information from Salesforce into excel.
Related Known Errors and Workarounds
Because the error is produced by Microsoft, we cannot prevent it from firing. The workaround is to accept the warning and the document opens as expected.