Arduino should be the default handler for *.ino *.pde. *.pde was used for pre-Arduino 1.0 code and I believe there's quite a bit more .pde files than .ino's still.
Arduino should be the default handler for *.ino *.pde. *.pde was used for pre-Arduino 1.0 code and I believe there's quite a bit more .pde files than .ino's still.