Fix language detected from content not applied if default language is set

Properly handle missing file extensions in `Buffer::setFileName`

Before 6263ce5, the user-preferred default language would apply to
files with no extension, but got overwritten by `L_TEXT` when the
extension was present *but unknown*.

This is why the `buf->_lang == L_TEXT` condition failed *only for
buffers with _no_ extension*.

Fix #11504, close #12286

PowerEditor/src/ScintillaComponent/Buffer.cpp

@@ -211,6 +211,8 @@ void Buffer::setFileName(const TCHAR *fn, LangType defaultLang)
 			newLang = nppParamInst.getLangFromExt(ext);
 		}
 	}
+	else if (!isUntitled()) // existing file with no extension
+		newLang = L_TEXT;
 
 	if (newLang == defaultLang || newLang == L_TEXT)	//language can probably be refined
 	{