init separated format controller, detect content pattern by format_filename extension or trailing slash availability

README.md

@@ -50,8 +50,10 @@ snac2nex -s /path/to/snac/storage -t /path/to/nex -u user1 -u user2
       --format-filename <FORMAT_FILENAME>
           Post filenames format
 
-          * escaped with `%%`
-          * be careful when removing seconds, as it may cause overwrite collisions
+          * append trailing slash to parse documents as the directory
+          * append `.gmi` or `.gemtext` extension to parse documents as the Gemtext
+          * escape with `%%` when using in `systemd` context
+          * be careful when removing the seconds part, as it may cause overwrite collisions!
 
           [default: %H-%M-%S]