I have released version 3.4.2, which should fix the bug.
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
I've forgot to add that the problem occurs only for text-mode transfers. So switch to binary and it should work fine. I hope that it also happens only for uploads. If you experience similar problem for downloads, please let me know ASAP. Thanks.
I'm not sure if there is a difference based on file size, but I was transferring a theme.php file for phpNuke and it added a lot of @^ characters where there was only whitespace before.
Is that a tab character? I don't really remember.