Yes, your visitors can upload files using your contact form, but only if you have a Gold plan or higher. Use the File upload field to do this. Go to the Form Editor and you’ll find the File Upload field under Basic Fields. Keep in mind, however, that form users cannot upload files bigger than 150 MB per File Upload field.
Form users can cancel the file they have uploaded by pressing the X sign next to the File Upload.
In My Account → Uploads Manager you’ll find all the files that have been uploaded by your form users. Read more on how to manage file uploads from online forms.