public interface FileUploadService {  @Multipart@POST("upload")Call<ResponseBody> upload(@Part("description") RequestBody description,@Part MultipartBody.Part file);
}

//先创建 service
FileUploadService service = retrofit.create(FileUploadService.class);//构建要上传的文件
File file = new File(filename);
RequestBody requestFile =RequestBody.create(MediaType.parse("application/otcet-stream"), file);MultipartBody.Part body =MultipartBody.Part.createFormData("aFile", file.getName(), requestFile);String descriptionString = "This is a description";
RequestBody description =RequestBody.create(MediaType.parse("multipart/form-data"), descriptionString);Call<ResponseBody> call = service.upload(description, body);
call.enqueue(new Callback<ResponseBody>() {@Overridepublic void onResponse(Call<ResponseBody> call,Response<ResponseBody> response) {System.out.println("success");}@Overridepublic void onFailure(Call<ResponseBody> call, Throwable t) {t.printStackTrace();}
});

Accept-Encoding: gzip
Content-Length: 470
Content-Type: multipart/form-data; boundary=9b670d44-63dc-4a8a-833d-66e45e0156ca
User-Agent: okhttp/3.2.0
X-Request-Id: 9d70e8cc-958b-4f42-b979-4c1fcd474352
Via: 1.1 vegur
Host: requestb.in
Total-Route-Time: 0
Connection: close
Connect-Time: 0

description: This is a description

–9b670d44-63dc-4a8a-833d-66e45e0156ca
Content-Disposition: form-data; name=”description”
Content-Transfer-Encoding: binary
Content-Type: multipart/form-data; charset=utf-8
Content-Length: 21This is a description
–9b670d44-63dc-4a8a-833d-66e45e0156ca
Content-Disposition: form-data; name=”aFile”; filename=”uploadedfile.txt”
Content-Type: application/otcet-stream
Content-Length: 32Visit me: http://www.println.net
–9b670d44-63dc-4a8a-833d-66e45e0156ca–

public interface FileUploadService {  @Multipart@POST("upload")Call<ResponseBody> upload(@Part("description") RequestBody description,//注意这里的参数 "aFile" 之前是在创建 MultipartBody.Part 的时候传入的@Part("aFile") File file);
}

–7d24e78e-4354-4ed4-9db4-57d799b6efb7
Content-Disposition: form-data; name=”description”
Content-Transfer-Encoding: binary
Content-Type: multipart/form-data; charset=utf-8
Content-Length: 21This is a description
–7d24e78e-4354-4ed4-9db4-57d799b6efb7
Content-Disposition: form-data; name=”aFile”
Content-Transfer-Encoding: binary
Content-Type: application/json; charset=UTF-8
Content-Length: 35// 注意这里！！之前是文件的内容，现在变成了文件的路径
{“path”:”samples/uploadedfile.txt”}
–7d24e78e-4354-4ed4-9db4-57d799b6efb7–

public interface Converter<F, T> {...abstract class Factory {...//返回一个满足条件的不限制类型的 Converterpublic Converter<?, ?> arbitraryConverter(Type originalType,Type convertedType, Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit){return null;}}
}

public <F, T> Converter<F, T> arbitraryConverter(Type orignalType,Type convertedType, Annotation[] parameterAnnotations, Annotation[] methodAnnotations) {return nextArbitraryConverter(null, orignalType, convertedType, parameterAnnotations, methodAnnotations);
}public <F, T> Converter<F, T> nextArbitraryConverter(Converter.Factory skipPast,Type type, Type convertedType,  Annotation[] parameterAnnotations, Annotation[] methodAnnotations) {checkNotNull(type, "type == null");checkNotNull(parameterAnnotations, "parameterAnnotations == null");checkNotNull(methodAnnotations, "methodAnnotations == null");int start = converterFactories.indexOf(skipPast) + 1;for (int i = start, count = converterFactories.size(); i < count; i++) {Converter.Factory factory = converterFactories.get(i);Converter<?, ?> converter =factory.arbitraryConverter(type, convertedType, parameterAnnotations, methodAnnotations, this);if (converter != null) {//noinspection uncheckedreturn (Converter<F, T>) converter;}}return null;
}

public class TypedFileMultiPartBodyConverterFactory extends Converter.Factory {@Overridepublic Converter<TypedFile, MultipartBody.Part> arbitraryConverter(Type originalType, Type convertedType, Annotation[] parameterAnnotations, Annotation[] methodAnnotations, Retrofit retrofit) {if (originalType == TypedFile.class && convertedType == MultipartBody.Part.class) {return new FileRequestBodyConverter();}return null;}
}

public class TypedFileMultiPartBodyConverter implements Converter<TypedFile, MultipartBody.Part> {@Overridepublic MultipartBody.Part convert(TypedFile typedFile) throws IOException {RequestBody requestFile =RequestBody.create(typedFile.getMediaType(), typedFile.getFile());return MultipartBody.Part.createFormData(typedFile.getName(), typedFile.getFile().getName(), requestFile);}
}

public class TypedFile {private MediaType mediaType;private String name;private File file;public TypedFile(String name, String filepath){this(name, new File(filepath));}public TypedFile(String name, File file) {this(MediaType.parse(MediaTypes.getMediaType(file)), name, file);}public TypedFile(MediaType mediaType, String name, String filepath) {this(mediaType, name, new File(filepath));}public TypedFile(MediaType mediaType, String name, File file) {this.mediaType = mediaType;this.name = name;this.file = file;}public String getName() {return name;}public MediaType getMediaType() {return mediaType;}public File getFile() {return file;}
}

...
} else if (MultipartBody.Part.class.isAssignableFrom(rawParameterType)) {return ParameterHandler.RawPart.INSTANCE;
} else {Converter<?, ?> converter =retrofit.arbitraryConverter(type, MultipartBody.Part.class, annotations, methodAnnotations);if(converter == null) {throw parameterError(p,"@Part annotation must supply a name or use MultipartBody.Part parameter type.");}return new ParameterHandler.TypedFileHandler((Converter<TypedFile, MultipartBody.Part>) converter);
}
...

static final class TypedFileHandler extends ParameterHandler<TypedFile>{private final Converter<TypedFile, MultipartBody.Part> converter;TypedFileHandler(Converter<TypedFile, MultipartBody.Part> converter) {this.converter = converter;}@Overridevoid apply(RequestBuilder builder, TypedFile value) throws IOException {if(value != null){builder.addPart(converter.convert(value));}}
}

public interface FileUploadService {@Multipart@POST("upload")Call<ResponseBody> upload(@Part("description") RequestBody description,@Part TypedFile typedFile);
}

etrofit retrofit = new Retrofit.Builder().baseUrl("http://www.println.net/").addConverterFactory(new TypedFileMultiPartBodyConverterFactory()).addConverterFactory(GsonConverterFactory.create()).build();FileUploadService service = retrofit.create(FileUploadService.class);
TypedFile typedFile = new TypedFile("aFile", filename);
String descriptionString = "This is a description";
RequestBody description =RequestBody.create(MediaType.parse("multipart/form-data"), descriptionString);Call<ResponseBody> call = service.upload(description, typedFile);
call.enqueue(...);