Adds an entry to the file list.
getFileDownloader("FileDownloaderID").DownloadFileAdd(ContentType, FileSize, fileName, Url);
A string with the MIME type of the file to download.
An integer equal to the file size in bytes. Specifying the file size is optional; to ignore it set the parameter to
A string with the name that will be used for saving the file.
A string with the URL of the file to download.
An integer equal to the index of the added entry in the file list.