How do I join files with an extension of this way: "spongebob.squarepants.avi._a?

looks like this: spongebob.squarepants.avi.__a spongebob.squarepants.avi.__b

You can do this in a command prompt with no external program: spongebob.squarepants.avi.__a copy / b + + spongebob.squarepants.avi.__b spongebob.squarepants.avi.__c spongebob.squarepants.avi / b / b / a each side says it is binary. avi file will spongebob.squarepants.avi.

Through the Fire and Flames – Dragonforce – Performed by Spongebob Squarepants.avi