Trouble opening attached CSV file

[Finner]

My online project includes 8 template files for customers that are in CSV format. I link to the CSV files in Flare. In past releases the files opened fine from the help target. Now, I get a message that says "The file format and extension of FILE.xls don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. do you want to open it anyway?" Users can still click Yes to open the file, but it would be better to get rid of that message. I've googled the issue and tried all the suggested fixes, but nothing gets rid of the message. Has anyone experienced this and been able to fix it?

Note: I get the same message when I click the file link in the PDF output.

[NorthEast]

If the file format is CSV, and the extension is XLS - an Excel spreadsheet, then the message is right because they don't match.

If you give the CSV files a .csv extension then presumably it will fix it?