Have you searched? We've discussed this a few times and demoed the hard/hacky version (uploading Base64-encoding images to a Textarea field) and the easy/recommended version (plugging UploadCare into the Marketo form)..
Sendsafely is another third party solution through which you can integrate single/multiple file upload functionality into your Marketo form. However, if you go with the third party API solution then the actual file will be uploaded to the third party server and you will only have uploaded file URL in the lead field.
If you want to upload and keep uploaded files in the Marketo then this can be done using Marketo Rest API.