Error while exporting to excel - File format and extension doesn't match

Still not clear. Did the code ever work for any excel file? Does it work for some and not others? Did it work for some cf version before your cf2018 update 19?

Also, even if you can't upload the file in this forum, you could put it on something like Dropbox, Google drive, one drive, or otherwise and give us the link. It may make it easier for us to come up with a solution, if it's at all possible for that file. 

Finally, as you may know, cf2018 is no longer updated since July 2023. If the cost of licensing cf2023 is an issue (because the 50% upgrade discount is only for those on cf2021), note that there's a special offer for a few more weeks where those on cf2018, cf2016, or earlier can buy a cf license at a 25% discount. More at https://www.carehart.org/blog/2024/7/8/limited_time_upgrade_discount_to_CF2023_from_older_releases

Example:

<cfheader name="Content-Disposition" value="attachment; filename=MySales.xls">
<cfcontent type="application/vnd.ms-excel">

<table border="1">
	<tr><td>Month</td><td>Quantity</td><td>$ Sales</td></tr>
	<tr><td>January</td><td>80</td><td >245</td></tr>
	<tr><td>February</td><td>100</td><td>699</td></tr>
	<tr><td>March</td><td>230</td><td >2036</td></tr>
	<tr><td>Total</td><td>=Sum(B2..B4)</td><td>=Sum(C2..C4)</td></tr>
</table>

You may then ignore Microsoft Office's prompt that the file you are trying to open is in a format different from the specified file extension.