[Java] Support for multipart upload other than java.io.File in Feign client

Description

Java Feign client generates endpoints with "multipart/form-data" payload in the following way (example from petstore sample):

ModelApiResponse uploadFile(@Param("petId") Long petId, @Param("additionalMetadata") String additionalMetadata, @Param("file") File file);

Notice the file parameter of type java.io.File in the api signature.

This seems counter-intuitive to me, as you may want to not only upload files stored on disk, but data that's just held in-memory.
Currently one may be forced to write the data to a file in order to call the generated method.

I'd suggest to declare byte[] instead, or at least provide an option for that in the config options.

Swagger-codegen version

2.3.1

Start with the generated PetApi.java example linked in the issue and inspect how Java Feign multipart/form-data parameters are represented. Determine how an in-memory upload type or configuration option should be exposed, then verify that generated client signatures support uploads without requiring java.io.File.