Will gin support multipart/mixed for uploading file?

[wccsama]

Description

As the code showing in the binding.go, there is no multipart/mixed supported.

How to reproduce

Expectations

$curl -i -X POST http://127.0.0.1:9702/upload -F "upfile=@/Users/test/Desktop/test.png" -F "type=image" -H "Content-Type: multipart/mixed"

Actual result

	ErrNotMultipart = &ProtocolError{"request Content-Type isn't multipart/form-data"}

Environment

• go version: 1.13