wp_generate_attachment_metadata gives me an empty array

Question

I have a custom post type and a metabox with a file input.

I can insert the attachment, but I can't update the attachment metadata and I don't know how to fix it because I don't receive any error.

Here is my code:

$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 ); 

echo $attach_id;
echo '<pre>';
print_r($filename);
echo '</pre>';
echo '<pre>';
print_r($attach_data);
echo '</pre>';

and here is the output:

96
Array
(
    [name] => one.png
    [type] => image/png
    [tmp_name] => /tmp/phphQ0e2v
    [error] => 0
    [size] => 144555
)
Array
(
)

As you can see, $attach_data is empty :(

Answer 1

From comment:

Let WordPress generate a file path and use that for the next steps:

$upload    = wp_handle_upload($filename, array('test_form' => false));
$attach_id = wp_insert_attachment( $attachment, $upload['file'], $post_id );
wp_update_attachment_metadata( $attach_id,  $attach_data ); 

Answer 2

This is, what finally fixed it for me:

apply_filters('wp_handle_upload', array(
    'file' => $file_path, 
    'url' => $file_url, 
    'type' => $file_type), 
'upload');

Explanation: I'm not quite sure why this fixed the error for me, but I assume that this either has something to do with plugins using the wp_handle_upload hook or that the filters add meta-data to the attachment, which otherwise would be missing in the wp_generate_attachment_metadata function.

Full function:

function add_to_media_lib($file_url, $file_path, $parent_post_id)
{
require_once(ABSPATH . 'wp-admin/includes/image.php');
require_once(ABSPATH . 'wp-admin/includes/file.php');

// Check the type of tile. We'll use this as the 'post_mime_type'.
$file_type = wp_check_filetype(basename($file_url), null);

// Get the path to the upload directory.
$wp_upload_dir = wp_upload_dir();

// Prepare an array of post data for the attachment.
$attachment = array(
    'guid' => $wp_upload_dir['url'] . '/' . basename($file_url),
    'post_mime_type' => $file_type['type'],
    'post_title' => preg_replace('/\.[^.]+$/', '', basename($file_url)),
    'post_content' => '',
    'post_status' => 'inherit',
    'post_parent' => $parent_post_id
);

// Insert the attachment.
$attach_id = wp_insert_attachment($attachment, $file_url, $parent_post_id);

// apply filters (important in some environments)
apply_filters('wp_handle_upload', array('file' => $file_path, 'url' => $file_url, 'type' => $file_type), 'upload');


// Generate the metadata for the attachment, and update the database record.
if ($attach_data = wp_generate_attachment_metadata($attach_id, $file_path)) {
    wp_update_attachment_metadata($attach_id, $attach_data);
} else {
    echo '<div id="message" class="error"><h1>Failed to create PDF-thumbnail Meta-Data</h1><pre>' . print_r($attach_data) . '</pre></div>';
}

return $attach_id;
}