After you created an expense (by clicking on 'Add an expense' under Expenses), you can upload a file fot this expense from Dropbox, Google Drive, OneDrive or your computer. For this, click on Upload in the details of the expense, or simply drag and drop the file you wish to upload in the Details panel of the expense.
You can of course choose not to upload any file or upload a file later.
To download a file you uploaded, simply click on it.