Example Plugin to show how to add a numeric/integer media settings field.

wpm_vie_settings_field.class.php

<?php
/**
 * Plugin Name: (WPM Vienna) Numeric Media Settings field
 * Description: Example Plugin to show how to add a numeric/integer media settings field.
 * Author:      Franz Josef Kaiser
 * Author URl:  http://unserkaiser.com
 * Plugin URl:  https://github.com/wecodemore/wcm_limitor
 * Version:     1.0
 * License:     MIT
 *
 * Copyright:   © Franz Josef Kaiser 2013
 */

register_uninstall_hook(    __FILE__, array( 'WPM_VIE_MS', 'on_uninstall' ) );
register_activation_hook(   __FILE__, array( 'WPM_VIE_MS', 'on_activation' ) );

add_action( 'plugins_loaded', array( 'WPM_VIE_MS', 'init' ) );
class WPM_VIE_MS
{
	/**
	 * Instance of the class
	 * @static
	 * @var    object
	 */
	protected static $instance;

	/**
	 * ID for the DB option
	 * @static
	 * @var    string
	 */
	public static $id = 'wpm_vie_int_field';

	/**
	 * Default for the DB option
	 * @static
	 * @var    int
	 */
	public static $default = 10;


	/**
	 * INIT - instantiates the class
	 * and checks if we're not in the middle of an import
	 * @static
	 * @return object|WPM_VIE_MS
	 */
	public static function init()
	{
		if ( defined( 'WP_LOAD_IMPORTERS' ) )
			return;

		null === self::$instance AND self::$instance = new self;
		return self::$instance;
	}

# =========== Setup/Install tasks

	/**
	 * Uninstall
	 * Deletes all options/media settings
	 * @static
	 * @return void
 	 */
	public static function on_uninstall()
	{
		if ( __FILE__ != WP_UNINSTALL_PLUGIN )
			return;

		delete_option( self::$id );
	}

	/**
	 * Activation
	 * Sets up defaults
	 * @static
	 * @return void
	 */
	public static function on_activation()
	{
		! get_option( self::$id )
			AND add_option(
				 self::$id
				,self::$default
			);
	}

# =========== Add actions


	/**
	 * Constructor
	 * @return \WPM_VIE_MS
	 */
	public function __construct()
	{
		add_action( 'adminmenu', array( $this, 'add_settings_field' ) );
	}


	/**
	 * Registers and adds settings fields for the
	 * Media Settings screen. Adds size limit fields.
	 * @wphook adminmenu
	 * @return void
	 */
	public function add_settings_field()
	{
		if ( 'options-media' !== get_current_screen()->id )
			return;

		$label = __( 'New Settings Field', 'wpm_vie_ms_textdomain' );
		register_setting(
			 'media'
			,'max_img_mp'
			,array( $this, 'sanitize_field' )
		);
		add_settings_field(
			 get_class( $this )
			,$label
			,array( $this, 'settings_field' )
			,'media'
			,'default'
			,array( 'label' => $label )
		);
	}


	/**
	 * Registers and adds settings fields for the
	 * Media Settings screen. Adds the WPM Vienna field.
	 * @wphook adminmenu
	 * @return void
	 */
	public function settings_field( $args )
	{
		$id = self::$id;
		?>
		<fieldset>
			<legend class="screen-reader-text">
				<span><?php echo $args['label']; ?></span>
			</legend>
			<input type="number"
				name="<?php echo $id; ?>"
				id="<?php echo $id; ?>"
				value="<?php form_option( $id ); ?>"
				step="1"
				min="0"
				class="small-text"
				/>
			<label for="<?php echo $id; ?>">
				<?php _e( 'WPM Vienna Avatar width', 'wpm_vie_ms_textdomain' ); ?>
			</label>
		</fieldset>
		<?php
	}


	/**
	 * Santize callback for the setting field.
	 * Returns only integers.
	 * @param  int $value
	 * @return int $value
	 */
	public function sanitize_field( $value )
	{
		return filter_var(
			 $value
			,FILTER_SANITIZE_NUMBER_INT
		);
	}
}