copyfile [-bytes num|-maxbytes num] fromFileId toFileId
Copies the rest of the file specified by fromFileId, starting from its current position, to the file specified by toFileId, starting from its current position.
If -bytes is specified, then num bytes are copied. If less than num bytes are available, an error is returned. If -maxbytes is specified, then num bytes are copied but no error is returned if less are available.
The command returns the number of bytes that were copied.
The -bytes option is particularly useful for mixing binary data in with ASCII commands or data in a data stream.