Hi Somboon Srisamanuwat,
I would suggest you to call the service while the form is saved.
If you are calling external service by adding a microlfow in On change or On Accept the file will be Saved in the External storage, but after uploading if you remove the file, the file which is uploaded is Unnecessary.
So I would suggest you to Call the service while saving the form which helps you witht Data Integrity and Unwanted calling as you have mentioned will also be eliminated.