Is it possible to allow customers to upload files using the Order Attributes M2 extension?
Yes, to provide such an option, you may create an Order Attribute with the Single file upload type:
In the attribute settings, it is possible to define the maximum file size for the uploaded file and the allowed file types.
Here is how the attribute looks on the frontend:

Since version 4.0.0 we added validation for files uploaded via the Single File Upload attributes (Please note: this update is backward incompatible as the mandatory attribute_code parameter was added to the uploadFile API endpoint)