If you are required by your client to submit any electronic attachments with your invoices (such as expense receipts, scanned copies of your paper invoices, etc.), you can do this after uploading your LEDES file (see How do I attach documents to my uploaded invoice?).
Make sure that the Submit invoices directly to the client without previewing option is NOT selected when you upload the invoice (see How do I access or change my firm's profile information? or How do I upload invoices?). If the option to submit the invoice without previewing is selected, your uploaded invoices are submitted to clients automatically and you are unable to preview invoices or add attachments.