Fixes#9966
Uses the validate function to paste images so that it concatenates the previous images.
FIxes the validate function to also accept data images.
Moves the size validation function to Utils to reuse the data path prefix constant and fix the possible crash when we try to get the size of a data image.
as we're dealing with local file URLs only here, we don't want to turn
the plus signs (`+`) into spaces (leads to failing to open the file,
emits an exception, crashes later on in our JS code)
Fixes#9538