In a seemingly endless quest to improve my sysadmin skills to a functional level, I had to learn to search for files without a given extension, in this case .conf.
find /path/to/files -type f ! -name "*.conf"
This finds all files in the given path (not recursively) that do NOT have the .conf extension, and ARE files.
The ! is used before the argument(s?) that you want to negate. e.g., moving the ! in front of the -type f argument (find . ! -type f -name "*.conf") will find all entries in the current directory, that are NOT files, but DO have the .conf extension.
To delete all the files in the current directory EXCEPT those with the .conf extension:
find . -type f ! -name "*.conf" -delete