alter-meta-rs-csv-importer.php

<?php

//github plugin url: https://github.com/hissy/rs-csv-importer
//wp plugin directory url: http://wordpress.org/plugins/really-simple-csv-importer/
//add to your functions.php in current child theme or create a child theme using any plugin/website


function really_simple_csv_importer_save_meta_filter( $meta, $post, $is_update ) {
	if( isset($meta['header_image']) ){
			include_once( ABSPATH . 'wp-admin/includes/image.php' );
			$imageurl =  $meta['header_image'];
			$imagetype = end(explode('/', getimagesize($imageurl)['mime']));
			$uniq_name = date('dmY').''.(int) microtime(true); 
			$filename = $uniq_name.'.'.$imagetype;

			$uploaddir = wp_upload_dir();
			$uploadfile = $uploaddir['path'] . '/' . $filename;
			$contents= file_get_contents($imageurl);
			$savefile = fopen($uploadfile, 'w');
			fwrite($savefile, $contents);
			fclose($savefile);

			$wp_filetype = wp_check_filetype(basename($filename), null );
			$attachment = array(
				'post_mime_type' => $wp_filetype['type'],
				'post_title' => $filename,
				'post_content' => '',
				'post_status' => 'inherit'
			);

			$attach_id = wp_insert_attachment( $attachment, $uploadfile );
			$imagenew = get_post( $attach_id );
			$fullsizepath = get_attached_file( $imagenew->ID );
			$attach_data = wp_generate_attachment_metadata( $attach_id, $fullsizepath );
			wp_update_attachment_metadata( $attach_id, $attach_data ); 
			
		$meta['header_image'] = $attach_id;
	}
    return $meta;
}
add_filter( 'really_simple_csv_importer_save_meta', 'really_simple_csv_importer_save_meta_filter', 10, 3 );