inc/class-Iac_Attach_Media.php
public static function get_thumbnail_file( $meta, $size = 'medium' ) {

		if ( ! isset( $meta[ 'sizes' ][ $size ] ) ) {
			$file = FALSE;
		} else {
			$dir = wp_upload_dir();
			$file_parts = array(
				$dir[ 'basedir' ],
				dirname( $meta[ 'file' ] ),
				$meta[ 'sizes' ][ $size ][ 'file' ]
			);

			$file = implode( DIRECTORY_SEPARATOR, $file_parts );
		}

		return apply_filters( 'iac_attach_media_thumbnail_file', $file, $meta, $size );
	}
public static function get_thumbnail_file( $meta, $size = 'medium' ) {

		if ( ! isset( $meta[ 'sizes' ][ $size ] ) ) {
			$file = FALSE;
		} else {
			$dir = wp_upload_dir();
			$file_parts = array(
				$dir[ 'basedir' ],
				dirname( $meta[ 'file' ] ),
				$meta[ 'sizes' ][ $size ][ 'file' ]
			);

			$file = implode( DIRECTORY_SEPARATOR, $file_parts );
		}

		return apply_filters( 'iac_attach_media_thumbnail_file', $file, $meta, $size );
	}
function to get the filesystem path to an image

@param string $size
@param array $meta (Return value of wp_get_attachment_metadata() )
@return string|false