Make @Part supporting filename property

[jeaking]

I am using Retrofit 2 in my application, it is great and very easy to learn. But I still met some issues when I tried to uploading files using Retrofit 2, the biggest one is that I didn't know how to specify the filename of uploading file. I finally got a solution by googling, it looks like this:

@POST("client/resources/upload/image")
@Multipart
Call<ResponseBody> upload(@Part("myFile\"; filename=\"myFile.png") RequestBody myFile); 

It work for me, but in my opinion this looks more like a work around, I think there should be another solution which is more elegant and straightforward.

I suggest to add a new property named "filename" to @part as following:

@documented
@target(PARAMETER)
@retention(RUNTIME)
public @interface Part {
String value();
String filename() default "";
/** The {@code Content-Transfer-Encoding} of this part. */
String encoding() default "binary";
}

Is this acceptable？Or any comments on this？

Thanks！

[JakeWharton]

This prevents the filename from being set dynamically which will make the feature not useful to most users. Please contribute discussion on #1140.