I am not sure whether it is a bug or my misunderstanding
I try to post binary files using metaWeblog.newMediaObject with b2evolution 2.4.0 "Venetian".

According to the specifications "bits" should be base64-encoded, something like that:

$fs = filesize($tmp_name);
$file = fopen($tmp_name, 'rb');
$data = fread($file, $fs);
$object = chunk_split(base64_encode($data));
fclose($file);

$request = new XML_RPC_Message('metaWeblog.newMediaObject', array(
                                 new XML_RPC_Value($blog_id),
                                 new XML_RPC_Value($username),
                                 new XML_RPC_Value($password),
                                 new XML_RPC_Value(array(
                                             'name' => new XML_RPC_Value($name),
                                             'type' => new XML_RPC_Value($type),
                                             'bits' => new XML_RPC_Value($object, 'base64')
                                             ), 'struct')
                                 ));

If this is correct, then the decoding in inc/xmlrpc/apis/_metaweblog.api.php is done incorrectly, the 'bits' are saved still base64-encoded into the media directory (and useless of course!)

I modified the code (see the decoding stuff below, inside // MVE comments) and it does the job. Still I am not 100% sure that I understood all right.

    $data = $contentstruct['bits'];

    // MVE 20080207 - isn't it supposed to be base64-encoded?
    $data = base64_decode($data);

    if ($data === FALSE) {
        return new xmlrpcresp(0, $xmlrpcerruser+9, // user error 9
                 'Cannot decode bits');
    }
    // MVE EOF

    $type = $contentstruct['type'];
    logIO( 'Received MIME type: '.$type );