As far as I can tell, the required property in the OpenAPI spec can be used to specify that an entire multipart/form-data part is required, but it cannot be used to indicate whether the filename directive is required.
This would be pretty useful since some multipart/form-data endpoints require the filename directive while others do not. If OpenAPI allowed specifying those semantics, then that could be used in various generators that consume OpenAPI (e.g. an SDK generator could check whether the end-user supplied a filename if it's required).