Attach My Upload is a WordPress plugin that allow to create an attachment post from a file already uploaded to uploads folder. Was wrote to answer a WPSE question here: http://wordpress.stackexchange.com/questions/141509/

amu.php

<?php
/**
 * Plugin Name: Attach My Upload
 * Description: Allow to create an attachment post from a file already uploaded to uploads folder.
 * Plugin URI: http://wordpress.stackexchange.com/questions/141509/
 * Author: G. M.
 * Author URI: http://wordpress.stackexchange.com/users/35541/g-m
 *
 */
/*
  Copyright (C) 2014 Giuseppe Mazzapica

  This program is free software; you can redistribute it and/or
  modify it under the terms of the GNU General Public License
  as published by the Free Software Foundation; either version 2
  of the License, or (at your option) any later version.

  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 */

namespace GM;

/**
 * Only in admin and only if not ajax
 */
if ( ! is_admin() || ( defined( 'DOING_AJAX' ) && DOING_AJAX ) ) {
    return;
}

/**
 * Require the class file, instantiate it and attach calss methods to proper hooks
 */
function initAMU() {
    require_once plugin_dir_path( __FILE__ ) . 'class_amu.php';
    $method = filter_input( INPUT_SERVER, 'REQUEST_METHOD', FILTER_SANITIZE_STRING );
    $amu = new AMU;
    if ( strtoupper( $method ) === 'POST' ) {
        $amu->save();
    }
    add_action( 'all_admin_notices', array ( $amu, 'form' ), PHP_INT_MAX );
}

/**
 * Let the game starts
 */
add_action( 'load-upload.php', 'GM\initAMU' );

class_amu.php

<?php

namespace GM;

class AMU {

    private $info = array ();

    /**
     * Run when upload.php is load from a POST method (i.e. a fome sumbit to upload.php)
     * After same checks (file existence, capability, nonce..) done by AMU::shouldSave() method
     * try to save the file sent via form as an attachment post, set AMU::$info property
     * according to results and finally attach to hook 'admin_notices' the method AMU::admin_notices()
     * that print a feedback to users
     *
     * @return void
     * @access public
     * @uses shouldSave()       Check data sent via form
     * @uses GM\AMU\create()    Create the attachment post
     */
    function save() {
        if ( ! $this->shouldSave() ) {
            return;
        }
        $file = filter_input( INPUT_POST, 'file_in_server', FILTER_SANITIZE_STRING );
        if ( ! empty( $file ) ) {
            remove_action( current_filter(), array ( $this, __FUNCTION__ ) );
            $id = $this->create( $file );
            if ( is_wp_error( $id ) ) {
                $this->info = array ( 'type' => 'error', 'msg' => $id->get_error_message() );
            } elseif ( is_numeric( $id ) ) {
                $url = self_admin_url( "post.php?post={$id}&action=edit" );
                $msg = sprintf( 'Attachment saved! <a href="%s">See it</a>.', esc_url( $url ) );
                $this->info = array ( 'type' => 'updated', 'msg' => $msg );
            }
            if ( ! empty( $this->info ) ) {
                add_action( 'admin_notices', array ( $this, 'admin_notices' ) );
            }
        }
    }

    /**
     * Print the form inside upload.php including file in plugin folder file that contain the markup
     *
     * @return void
     * @access public
     */
    function form() {
        if ( get_current_screen()->id !== 'upload' ) {
            return;
        }
        remove_action( current_filter(), array ( $this, __FUNCTION__ ) );
        $wp_upload_dir = wp_upload_dir();
        $base = trailingslashit( $wp_upload_dir['basedir'] );
        require plugin_dir_path( __FILE__ ) . 'form.php';
    }

    /**
     * Print admin notices when form is submitted giving feedback to users
     *
     * @return void
     */
    function admin_notices() {
        if ( current_filter() !== 'admin_notices' ) {
            return;
        }
        remove_action( current_filter(), array ( $this, __FUNCTION__ ) );
        $format = '<div class="%s"><p>%s</p></div>';
        if ( empty( $this->info ) ) {
            return;
        }
        if ( ! isset( $this->info['type'] ) ) {
            $this->info['type'] = 'error';
        }
        if ( ! isset( $this->info['msg'] ) ) {
            $this->info['msg'] = 'Unknown';
        }
        printf( $format, $this->info['type'], $this->info['msg'] );
        $this->info = array ();
    }

    /**
     * Get a relative path to a file and create an attachment post
     *
     * @param string $file  Relative path to 'uploads folder' for the file
     * @return int          The attachment ID, 0 on failure
     * @access private
     * @uses shouldCreate
     */
    private function create( $file = '' ) {
        $wp_upload_dir = wp_upload_dir();
        $fullpath = $this->shouldCreate( $file, $wp_upload_dir );
        if ( is_wp_error( $fullpath ) ) {
            return $fullpath;
        }
        $filetype = wp_check_filetype( basename( $fullpath ), NULL );
        $attachment = array (
            'guid'           => $wp_upload_dir['url'] . '/' . basename( $fullpath ),
            'post_mime_type' => $filetype['type'],
            'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $fullpath ) ),
            'post_content'   => '',
            'post_status'    => 'inherit'
        );
        $attach_id = wp_insert_attachment( $attachment, $fullpath, 0 );
        require_once( ABSPATH . 'wp-admin/includes/image.php' );
        $attach_data = wp_generate_attachment_metadata( $attach_id, $fullpath );
        wp_update_attachment_metadata( $attach_id, $attach_data );
        return $attach_id;
    }

    /**
     * Check current user and nonce to see if to going on saving the file sent via form
     * @return boolean
     */
    private function shouldSave() {
        if ( ! current_user_can( 'upload_files' ) ) {
            return FALSE;
        }
        if ( ! check_admin_referer( 'file_in_server', '_n' ) ) {
            return FALSE;
        }
        return TRUE;
    }

    /**
     * Check existence of the file and using AMU::isAttached() check if it is already attached to
     * an existend attachemnt post. Return fullpath of the file if both check are passed.
     *
     * @param string $file          Relative path to 'uploads folder' for the file
     * @param array $wp_upload_dir  Whatever returned by wp_upload_dir()
     * @return string|\WP_Error     Full path of the fail or WP_Error on failure
     */
    private function shouldCreate( $file, $wp_upload_dir ) {
        if ( ! is_string( $file ) || empty( $file ) ) {
            $msg = sprintf( "Please use valid file relative path with %s", __FUNCTION__ );
            return new \WP_Error( 'bad_file_arg', $msg );
        }
        $base = $wp_upload_dir['basedir'];
        $fullpath = trailingslashit( $base ) . $file;
        if ( ! file_exists( $fullpath ) ) {
            $msg = sprintf( "The file %s does not exists", $fullpath );
            return new \WP_Error( 'file_not_exists', $msg );
        }
        $already = $this->isAttached( $file );
        if ( $already > 0 ) {
            $url = get_edit_post_link( $already, '' );
            $link = sprintf( '<a href="%s">(ID: %d)</a>', $url, $already );
            $msg = sprintf( "The file %s is already connected to an attachment %s", $file, $link );
            return new \WP_Error( 'file_attached', $msg );
        }
        return $fullpath;
    }

    /**
     * Query database to see if a file is already attached to an attachement post
     *
     * @global wpdb $wpdb
     * @param string $file  Relative path to 'uploads folder' for the file
     * @return int          Attachement post id if found, 0 otherwise
     */
    private function isAttached( $file ) {
        global $wpdb;
        $query = "SELECT p.ID FROM {$wpdb->posts} p "
            . "INNER JOIN {$wpdb->postmeta} m ON p.ID = m.post_id "
            . "WHERE p.post_type = 'attachment' "
            . "AND m.meta_key = '_wp_attached_file' "
            . "AND m.meta_value = %s GROUP BY p.ID LIMIT 1";
        return (int) $wpdb->get_var( $wpdb->prepare( $query, $file ) );
    }

}

form.php

<?php if ( ! defined( 'ABSPATH' ) || ! isset( $base ) ) exit(); ?>
<div class="wrap">
    <h3>Convert an uploaded file to a media post
        <a href="#" id="hide-attachment_from_server" class="add-new-h2">+</a>
    </h3>
    <form method="post" style="display:none;">
        <?php wp_nonce_field( 'file_in_server', '_n' ); ?>
        <table class="form-table">
            <tbody>
                <tr>
                    <td>
                        <label for="attachment_from_server">
                            <strong>File path:</strong>
                        </label><br>
                        <?php echo $base ?>
                        <input name="file_in_server" value="" class="regular-text code" type="text">
                    </td>
                </tr>
            </tbody>
        </table>
        <p class="submit">
            <input name="submit" class="button button-primary" value="Create Attachment" type="submit">
        </p>
    </form>
    <hr>
</div>
<script>
    jQuery(document).on('click', '#hide-attachment_from_server', function(e) {
        e.preventDefault();
        var $btn = jQuery(this);
        var $form = jQuery(this).closest('div.wrap').find('form');
        if ($form.is(':visible')) {
            $form.hide();
            $btn.text('+');
        } else {
            $form.show();
            $btn.html('&times;');
        }
    });
</script>

Thank you! :)

How to upload all file from a directory? I have hundred of images to import. Please, help me.

It seems that WP 3.9.1 has a problem with line 28 on amu.php.

@Elbatron I guess problem with that line is you version of PHP. Namespaces are supported in PHP 5.3+

thanks