text: Title: Specify `filename` and `content_type` headers through `Part` annotation
  Body: **Is your feature request related to a problem? Please describe.**
  Related to a problem surfaced by @Shanoir on [Gitter](https://gitter.im/python-uplink/Lobby?at=5d9f6234eac5612d22ec000f).
  
  **Describe the solution you'd like**
  I think that it would be a good idea for us to let users set the filename and content-type headers through the Part annotation. Maybe like so: 
  
  ```python
      @multipart
      @retry(when=raises(Exception) | status(400),backoff=fixed(4),stop=after_attempt(3))
      @headers(headersCollector)
      @post("api/v1/data/file-upload")
      def uploadFile(self, file: Part(filename="data.edl", content_type="application/octet-stream"), serialNumber: Field, append: Field): pass
  ```
  