How can I add a custom meta value on file upload?

Question

I am looking for an option where a custom meta value will be saved when user uploads an attachment.

Lets say I need a meta key _example_meta_key and want to save meta value of this as ex087659bh (It will be a randomly generated number, I can take care of this.)

The problem is, I can't seem to find any filters to add custom meta value on upload.

There are some tutorials which shows the way to add fields in media edit form but I need this to be executed at the time of file upload.

Answer 1

Use added_post_meta and update_post_meta with the $post_id. For extended properties see this post and this for more image functions.

add_action('added_post_meta', 'wpse_20151218_after_post_meta', 10, 4);

function wpse_20151218_after_post_meta($meta_id, $post_id, $meta_key, $meta_value) {

    // _wp_attachment_metadata added
    if($meta_key === '_wp_attachment_metadata') {

        // Add Custom Field
        update_post_meta($post_id, '_example_meta_key', 'ex087659bh');

        // _wp_attached_file
        // _wp_attachment_metadata (serialized)
        // _wp_attachment_image_alt
        // _example_meta_key

        $attachment_meta = get_post_meta($post_id);
    }
}

Answer 2

As @jgraup pointed out. You can use the filter wp_handle_upload. The final code will look like something what is pasted below. You might want to use only one of the two hooks or maybe both of them.

add_filter('wp_handle_upload_prefilter', 'startOfWpHandleUpload', 10, 2 );

function startOfWpHandleUpload( $file ) {

    /* Here you can access the following properties of the uploaded file
     * before it is processed by the wp_handle_upload function. 
     */

    /* Name of the uploaded file. */
    $file['name'];
    /* MIME Type of the uploaded file. */
    $file['type'];
    /* Temporary location of the stored file. */
    $file['tmp_name'];
    /* "0" for no errror. Don't know what is for error. */
    $file['error'];
    /* size in bytes of the uploaded file. */
    $file['size'];

    return $file;
}

add_filter('wp_handle_upload', 'endOfWpHandleUpload', 10, 2 );

function endOfWpHandleUpload( $file, $type ) {

    /* Here you can access the following properties of the uploaded file
     * before it is processed by the wp_handle_upload function.
     */

    /* Absolute path of the file on the filesystem of the operating system. */
    $file['file'];
    /* Public accessible URL of the file over the Internet. */
    $file['url'];
    /* MIME type of the file. */
    $file['type'];

    return $file;
}