You can configure the add-on to add multiple attachments. Add-on can add files only from Google Drive. Add-on automatically converts Google Documents, Spreadsheets and Slides to PDF files when those files are being attached.
You can configure attachment names - attachment names will be the same as names of files in Google Drive, you can choose a column with attachment names or you can set a name.
Be very careful when setting attachment names from a column or you set an attachment name. Always make sure the file type of a file being attached and the attachment name extension match. Otherwise you can attach files with wrong extension and users will not be able to open attached files!
KEEP IN MIND: Certain file types cannot be attached as they might carry harmful content.