file/sendExisting - Sends an existing file.

Method parameters:
id - The ID of the file to send.
receiver_username - The Zvini username of the receiver.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
FILE_NOT_FOUND - A file with the ID doesn't exist.
ENTER_RECEIVER_USERNAME - The receiver username is empty.
INVALID_RECEIVER_USERNAME - The receiver username is invalid.
RECEIVER_NOT_FOUND - No such receiver with the username.
RECEIVER_NOT_RECEIVING - The receiver hasn't opened a connection to receive files from you.