Cant move file after media_handle_upload

Question

i am trying to create an meta image uploader for a a custom post type with media_handle_upload() but it only returns an error teling me that it could not move the image to the folder "wp-content/year/month" - regular uploads do work without problems, there is should not be a directory permission problem...

I was using wp_insert_attachment before, but then my uploads had no thumbnails inside the media library for some reason, also the WP Codex stated that I should use media_handle_upload to save additional function calls. Can anybody help? This is my code:

/// SAVE MITARBEITER META DATA
function mitarbeiter_save_post_meta($post_id,$post){
    /// GET THE POST TYPE OBJECT
    $post_type = get_post_type_object( $post->post_type );

    /// CHECK IF THE CURRENT USER HAS THE NEEDED PERMISSION
    if ( !current_user_can( $post_type->cap->edit_post, $post_id ) )
        return $post_id;

    /// PREVENT CLEARING THE CUSTOMFIELDS BY AUTOSAVE, QUICK EDIT OR BULK EDIT
    if ((defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) || (defined('DOING_AJAX') && DOING_AJAX) || isset($_REQUEST['bulk_edit']))
        return;

    /// UPDATE DATA
    $mitarbeiter_leitung = (isset($_POST['mitarbeiter_leitung'])) ? $_POST['mitarbeiter_leitung'] : "";
    $mitarbeiter_email = (isset($_POST['mitarbeiter_email'])) ? $_POST['mitarbeiter_email'] : "";
    $mitarbeiter_position = (isset($_POST['mitarbeiter_position'])) ? $_POST['mitarbeiter_position'] : "";
    $mitarbeiter_position_extra = (isset($_POST['mitarbeiter_position_extra'])) ? $_POST['mitarbeiter_position_extra'] : "";
    $mitarbeiter_position_EN = (isset($_POST['mitarbeiter_position_EN'])) ? $_POST['mitarbeiter_position_EN'] : "";
    $mitarbeiter_position_extra_EN = (isset($_POST['mitarbeiter_position_extra_EN'])) ? $_POST['mitarbeiter_position_extra_EN'] : "";
    update_post_meta( $post_id, 'mitarbeiter_leitung', $mitarbeiter_leitung );
    update_post_meta( $post_id, 'mitarbeiter_email', $mitarbeiter_email );
    update_post_meta( $post_id, 'mitarbeiter_position', $mitarbeiter_position );
    update_post_meta( $post_id, 'mitarbeiter_position_extra', $mitarbeiter_position_extra );
    update_post_meta( $post_id, 'mitarbeiter_position_EN', $mitarbeiter_position_EN );
    update_post_meta( $post_id, 'mitarbeiter_position_extra_EN', $mitarbeiter_position_extra_EN );

    /// CUSTOM IMAGE UPLOAD
    if(!empty($_FILES['custom_upload']['name'])){ //New upload
        require_once( ABSPATH . 'wp-admin/includes/file.php' );
        $override['action'] = 'editpost';
        $uploaded_file = wp_handle_upload($_FILES['custom_upload'], $override);
        $post_id = $post->ID;
        $attachment = array(
            'post_title' => $_FILES['custom_upload']['name'],
            'post_content' => '',
            'post_type' => 'attachment',
            'post_parent' => $post->ID,
            'post_mime_type' => $_FILES['custom_upload']['type'],
            'guid' => $uploaded_file['url']
        );
        #$id = wp_insert_attachment( $attachment,$_FILES['custom_upload'][ 'file' ],$post->ID);
        #wp_update_attachment_metadata( $id, wp_generate_attachment_metadata( $id, $_FILES['custom_upload']['file'] ) );
        $id = media_handle_upload('custom_upload',$post->ID);
        update_post_meta($post->ID, "custom_upload",$id);
    }
}
add_action( 'save_post', 'mitarbeiter_save_post_meta', 10, 2 );

/// MAKE FILEUPLOAD POSSIBLE
function fileupload_metabox_header(){
    ?>
    <script>
    jQuery(document).ready(function(){
        // Das ist ein Test
        jQuery("form#post").attr("enctype","multipart/form-data");
        jQuery("form#post").attr("encoding","multipart/form-data");
    });
    </script>
    <?php
}
add_action('admin_head', 'fileupload_metabox_header');

Answer 1

You're doing it twice - media_handle_upload is a wrapper for wp_handle_upload.

Just use the former like so:

require_once ABSPATH . 'wp-admin/includes/file.php';
$id = media_handle_upload(
    'custom_upload',
    $post->ID,
    array(), // Attachment post data overrides, WordPress will do all the grunt work by default
    array(
        'test_form' => false,
        'action' => 'editpost',
    )
);

if ( ! is_wp_error( $id ) )
    update_post_meta( $post->ID, 'custom_upload', $id );