write a file with some code, save it as .c, then form the menu, "save as" .cpp. the file type in "Documents" is still C source file.
should have been changed to c++ source file.
geany 1.22 on x86 fedora 17.
The behaviour is correct, looking at your better description of your actual actions on IRC, you just saved the file without changing the filetype.
Filetypes are not just dependent on extensions, they can be set from mode lines in the file and set manually by users. Because it does not know what set the filetype, just changing the extension without closing the file means Geany retains the current filetype.
If you close and re-open the file Geany will determine the filetype again or you can set the filetype from the document menu without closing.