After post a "multipart/form-data" form, $HTTP_POST_VARS is not published and remains empty, which causes b2evolution doesn't upload the file correctly.
Initially I thought if I use PHP4, the problem won't be there. After install PHP4, I found the bundled MySQL Client (3.23) doesn't work with MySQL Server 4.1!
File a bug report at php.net (bug#32812), you can check its status here.
I finally decided to keep the same version of PHP, and wait for the fix.