hey, I just found your post when I was searching for info on encodings. I see nobody has responded, which is disappointing.
I don't know if it'll work for you, but I have a suggestion. This seems to work for me on V16. I had to do a bit of experimentation since the info on encodings in the help file are outdated and incorrect.
My use case is editing php and html files that are utf8 but should not have a BOM. This should work for your yml files too I believe.
The help mentions Auto XML, Auto EBCDIC, etc but makes no mention of Auto HTML, so I don't know what that should do exactly, but Auto HTML is default for html extension, whereas Automatic is default for htm, php and php3 extensions. I tried setting them all to Auto HTML but that did not work. Since I don't expect to edit any non-UTF8 files, I tried changing the encoding for the php extension to UTF-8. Still, it does not work. Dropping the file on SlickEdit, it opens in wrong mode (but as you know, which mode is anyone's guess).
There is a hint in the help file. It states that the mode will be remembered for each file. If you go through the File | Open dialog and explicitly select UTF-8 from the encodings, the file does open in UTF-8 mode. Close the file, then open again via drag'n'drop instead of the Open dialog. Now, finally, the file is open in UTF-8 mode and saving it does not trash the contents. I tested this on files that contain three languages that would use three different code pages (West Europe 8859-1, East Europe 8859-2, and Russian 8859-??) if it weren't for Unicode.
I hope this helps you and anyone else facing the same problem. It would be nice if SlickEdit help section on encoding was updated to document soe of the addition Auto* encodings. It would be nicer if settings the encoding in the file association manager actually worked. It seems to do jack sh!t when I change the settings there. It would also be nice if there was some option to not write the BOM when saving UTF-8 files. I added more files to this web project and had to go remove the BOM with a hex editor. For bonus surprise fun, when I do that, the file goes back to opening as ascii until I manually File | Open with UTF-8 encoding explicitly set, then SlickEdit re-remembers the encoding.
[Me: Visual SlickEdit user since v2 on OS/2, disappointed when OS/2 was dropped, unsure what market there is for HPUX version now if there was no perceived OS/2 market. Loyal SlickEdit user despite having to use it on Windows for years. Disappointed by the lack of innovation ever since the version number acceleration set in around V6 or so. I can skip a few major versions between upgrades and then see no real feature difference when I make the jump, but the basic problems I've put up with for ages are still there (2GB file limit even on 64bit builds? WTF) yet still using it because everything else is worse.]