Return http form post extension (OpenSubsonic) (#2676)

server/subsonic/opensubsonic.go

@@ -10,6 +10,7 @@ func (api *Router) GetOpenSubsonicExtensions(_ *http.Request) (*responses.Subson
 	response := newResponse()
 	response.OpenSubsonicExtensions = &responses.OpenSubsonicExtensions{
 		{Name: "transcodeOffset", Versions: []int32{1}},
+		{Name: "formPost", Versions: []int32{1}},
 	}
 	return response, nil
 }