When you say upload, I assume from your mention of a container field that you really mean inserting a file/picture into a container field.
You can import multiple items (eg. pdf or image files) using the Import Records script step and specifying a folder as the data source (see screenshot). then in the import map point to your container field. If you follow this approach you can import your folderful of invoices and FM will create a new record for each invoice with the invoice stored in the container field.