How to state parameter requirements in HTTP headers?

For example, as described in the guide, in a request such as

POST /upload HTTP/1.1
Content-Length: 428
Content-Type: multipart/form-data; boundary=abcde12345
--abcde12345
Content-Disposition: form-data; name="profileImage"; filename="image1.png"
Content-Type: application/octet-stream
{…file content…}
--abcde12345--

how would a spec state that the filename parameter in header Content-Disposition is required and explain special usage in a description?
RFCs state that the filename parameter is optional.
It's not clear how to express parameters for a header object.

Start with the linked multipart request guide and the OpenAPI Header Object definition, then compare the question with RFC 7578's treatment of the filename parameter. Determine whether the specification can express this requirement and description, and document the agreed syntax or limitation with a representative example.