Your code sample is not unit testable because it attempts to test the HTTP protocol which is already well tested and known to work properly. However, you can test the MultiPartMimeFileUpload method using an integration test. The integration test will use an HTTP client to invoke the MultiPartMimeFileUpload action. Examples of HTTP clients are PostMan, the C# HttpClient, a browser, etc.
If the intent is to unit test file upload then replace the HttpRequestMessage parameter with IFormFile. Mock the IFromFile to include whatever data is under test.
How to mock an IFormFile for a unit/integration test in ASP.NET Core?