It is intended that saving a file under a different name does not change the filetype. Filetypes are only automatically detected when a file is first opened, and after that it is not changed by Geany since a user can set it from the `Document` menu.
Example use-cases are scripts embedded in HTML and the user selects the filetype to be the script language so its highlighted and parsed, even though the filename is still `.html`, or cases where Geany detects the filetype wrongly and the user sets it. It would be annoying if the filetype kept changing.
It might be possible to have an option in the Save As dialog, but clicking that is not much of a saving from `Document->Set Filetype->select type` before or after saving.