Hi Lucjan,
We've just finished the feature and the code is available on Github. Please download and try it.
In a couple of days, I'll release a new version of Meta Box and write a tutorial about that.
To upload to custom folder, please use file
field and set a new attribute upload_dir
to the path of the new folder, like this:
array(
'type' => 'file',
'id' => 'f',
'name' => 'File',
'upload_dir' => '/path/to/your/folder/', // You can use ABSPATH . '/downloads/'
)