Server side web service adds a "=" (equals) sign at the ends of the data sent as body

Android app turns a JSON file to a String, encodes it into bytes according to UTF-8 and then sends it using an output stream. However, the body received by the server has an extra “=” (equal) sign at the end of it when it is decoded according to UTF-8. Why does this happen?

I reduced the length of string by one by deleting the last character.