Kate is using now ~/.kde/share/config/katemoderc
From GUI: Kate menu → Settings → Configure Kate... → Editor Component → Open/Save → Modes & Filetypes
Select filetype:
Sources/Fortran, add;*.hto file extensions, raise priority from9to15(Should be ok, Max priority for.hwas inSources/C++which is9)

- Close then reopen the files.
Close then reopen the files.