In Questionnaires, Proposals, and Lead Capture forms your clients can upload files! Drag and drop that FILE UPLOADER element into your form and it allows them to upload away.
If you click on that uploader in edit mode it will pop up with options. If you only want you clients to be able to upload one item or images only, you can toggle that here:
Once in your form, your clients have the option to upload as many files as they want. Limit per file is 5mb.