Yummy FTP seems to loose the association between file and editor. The feature I originally told about is to associate one file to an editor using extension in addition to file by file.
In preferences panel just say "ok, for .css is CSSedit, for .php, espresso and so on.
For now this is not working. For example I quit Yummy FTP and for a CSS file I was working on this morning the association to CSSEdit I made not exist anymore. Maybe this is a bug !
Actually, you're right! It only happens if you choose 'Other' then select your own choice. If you choose the Default setting, or one of the predefined options then the setting is saved.
Anyway, now all is fixed in the next build. Thanks for reporting the bug