Specify `filename` and `content_type` headers through `Part` annotation

[prkumar]

Is your feature request related to a problem? Please describe. Related to a problem surfaced by @Shanoir on Gitter.

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:

    @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

[leiserfg]

You can pass it if you use the Part as a tuple (at least with request as backend) like:

client.upload(field_name=('data.json', data))

a new implementation should not break that behavior (at least without providing a workaround) because there are APIs that expect a meaningful filename.