When attempting a download of a export via download output option certain exports will download as a .ashx instead of a .xlsx.
When viewing the records in the exported file we see some have "special characters". These have been either pasted into text fields or have been added via a conversion. Users can review the file via Firefox where the file can open be opened to see what hex-code error it produces. This will allow you to track down the offending characters via software such as notepad ++ and remove them from the text fields.
Steps to Duplicate
Attempt to download output a file that has special hex-code characters inserted into it via pasting or converted data.