You can upload multiple files under the Finances section. When you get to the Finances section, you'll be asked to select your main income type, then you'll be able to upload as many documents to your application as you wish. If you'd like to explain your financial situation in more detail, we'd advise you to write a message in a Word document and upload it to your application. Please read this article for guidance on writing and uploading a note.
If you've already submitted your application, you'll need to share additional documents with Let Alliance via the app. Go to Support > Create new message > Referencing > Share additional documents.