As of approx 21st Jan 2021, webform downloads in Excel format can no longer be opened in Excel, on several sites. Excel says:
"Excel cannot open the file ’blah.xlsx’ because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file."
This is happening on multiple sites, where previously the downloads could be opened OK in Excel. The timing fits with the Drupal core 7.78 update, however I have replicated the issue on old development copies of two of the sites on earlier Drupal versions: 7.69 and 7.75 . That seems to rule out the core update as the cause. There were no other updates common to all the affected sites in the relevant time frame. I've replicated the problem on different hosting environments.
Another possibility might be if there's been a recent Excel update that leads to these files being reported as corrupt when previously they were accepted.
Has anyone else seen this issue?
Drupal core versions: 7.69, 7.75, 7.78
Webform versions: 7.x-4.21 and 7.x-4.23