Empty METADATA for a received image using TweetPress API

Well, the title explains it all. I do have problems getting the metadata filled in wp_postmeta. This only appears when I try to add an attachment (image) using the TweetPress API (http://brandontreb.com/tweetpress/tweetpress-api/). After receiving the image (send with Twittelator for iPhone) I save the image to ../wp-content/uploads/ using

move_uploaded_file($_FILES['media']['tmp_name'], $uploadFile)

The file is correctly copied to the desired folder, with the permission set to 644. After this step I make the desired thumbs and smaller versions of the image using:

image_resize( $resizefile, $tthumb ['w'], $tthumb ['h'], $tthumb ['c'])

The smaller copies of the image are correctly created and placed in ../wp-content/uploads/, with permision set to 666.

So far all goes fine. At this moment I try to add the image to the wp_post & wp_postmeta file using:

$wp_filetype = wp_check_filetype(basename($fileName), null );
$attachment = array(
   'post_mime_type' => $wp_filetype['type'],
   'post_title' => preg_replace('/\.[^.]+$/', '', basename($filename)),
   'post_content' => '',
   'post_author' => '1',
   'post_title' => $_POST['message'],
   'post_name' => $_POST['message'],
   'guid' => get_option ( 'siteurl' ) . '/wp-content/uploads/' . $fileName,
   'post_status' => 'inherit'
);

$attach_id   = wp_insert_attachment( $attachment, $fileName, $post_id );
$attach_data = wp_generate_attachment_metadata( $attach_id, $fileName );
wp_update_attachment_metadata( $attach_id,  $attach_data );

The attachment is correctly inserted into wp_post. Even the records made into wp_postmeta do exists. The only problem is the record with the “_wp_attachment_metadata” key. This one only contains “a:0:{}“, instead of the expected long string. Because of this “empty” string I’m not able to edit the image inside WordPress. I get the error “Image data does not exist. Please re-upload the image.”. Also WordPress thinks there are no thumbnails, which do exists.

Who can help me out? Why is there no metadata? Are my steps correct? Thanks in advance.