corrupted attachvent by base64_encode

Hi, All! I have a problem with sending binary attachment with mail() function. To do so I use standart scheme: chunk_split(base64_encode(fread($f,filesize($filename)))). The system is FreeBSD4.7, Apache2, PHP 4.3.4 When the attachment is plain text file , it passes well. By when I use excell, zip and so on, the attachment is corrupted. I tried various scripts which I found in the internet, so I don't think that the problem is in my code. It's looks like the base64_code() function makes something wrong. Or something wrong is in my system tunes. Maybe php.ini. I sow in various forums questions like this one. But there was no answers on them. Please , explain me what I do wrong . Thank you.