There is only one parameter that is required for creating an attachment: owners. You must specify at least one owner of type record or form. If you create a record attachment you can optionally include a name and file_size. The name will be the name of the file shown in the record information. The file_size is only used for verifying that uploading this attachment will not exceed your current storage limit. If no file_size is provided the attachment may be rejected once it has been uploaded. The response will provide the url to upload (PUT) the file to.