Export Query Result > As CSV File..
When I customise the filename, the preview shows the .csv extension, but the file gets saved without the extension. The current work around is to add a trailing '.' (period) to the customised name which results in a file 'xxx-xxx-xxx-phlebo..csv'
SequelPro Version: Build 5200 (67649f0)
Also reproducible on the current stable build.