[RETROFIT2] Field string/binary not work correctly with retrofit2 library.

[Arodrigal]

Description

I am working in a Java environment. Java projects build the model and the libraries by connection/client type (feign, retrofit2, etc) with Swagger Codegen.
In any case, I have an API Rest service with a file field defined in the following way:
"type": "string",
"format": "byte"
The generated libraries have the corresponding model class with file field type byte[] but the same field in the Rest service class is generated with string type. This generates a casting error because the generated client doesn’t know how to convert a Base64 string to a byte[].

The retrofit2 library generates a JSON.java file with date adapters but ByteArrayAdapter is not added.

Swagger-codegen version

2.4.7

Swagger declaration file content or url

avatar:
	type: string
	format: binary

Command line used for generation

<profile>
<id>AndroidClientGeneration</id>
<dependencies>
	<dependency>
		<groupId>com.squareup.retrofit2</groupId>
	<artifactId>retrofit</artifactId>
	<version>2.5.0</version>
	</dependency>
	<dependency>
		<groupId>com.squareup.retrofit2</groupId>
		<artifactId>converter-gson</artifactId>
		<version>2.5.0</version>
	</dependency>
	<dependency>
		<groupId>com.squareup.retrofit2</groupId>
		<artifactId>converter-scalars</artifactId>
		<version>2.5.0</version>
	</dependency>
	<dependency>
		<groupId>com.google.code.gson</groupId>
		<artifactId>gson</artifactId>
		<version>2.8.5</version>
	</dependency>
	<dependency>
		<groupId>io.gsonfire</groupId>
		<artifactId>gson-fire</artifactId>
		<version>1.8.3</version>
	</dependency>
	<dependency>
	<groupId>org.threeten</groupId>
	<artifactId>threetenbp</artifactId>
	<version>1.4.0</version>
</dependency>
</dependencies>
<build>
	<plugins>
		<plugin>
			<groupId>io.swagger</groupId>
			<artifactId>swagger-codegen-maven-plugin</artifactId>
			<version>${swagger.codegen.version}</version>
			<executions>
				<execution>
					<phase>generate-sources</phase>
					<goals>
						<goal>generate</goal>
					</goals>
					<configuration>
						<inputSpec>${api-definition}</inputSpec>
						<output>${generated-android-client-sources}</output>
						<ignoreFileOverride>${ignore-file-override}</ignoreFileOverride>
						<language>java</language>
						<library>retrofit2</library>
						<configOptions>
							<apiPackage>${package-client-codegen}.api</apiPackage>
							<modelPackage>${package-client-codegen}.model</modelPackage>
							<invokerPackage>${package-client-codegen}</invokerPackage>
							<licenseName>private</licenseName>
							<serializableModel>true</serializableModel>
							<useBeanValidation>true</useBeanValidation>
							<dateLibrary>threetenbp</dateLibrary>
						</configOptions>
					</configuration>
				</execution>
			</executions>
		</plugin>
		<plugin>
			<groupId>org.codehaus.mojo</groupId>
			<artifactId>build-helper-maven-plugin</artifactId>
			<executions>
				<execution>
					<id>add-generated-source</id>
					<phase>initialize</phase>
					<goals>
						<goal>add-source</goal>
					</goals>
					<configuration>
						<sources>
							<source>${generated-sources}</source>
						</sources>
					</configuration>
				</execution>
			</executions>
		</plugin>
		<plugin>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-maven-plugin</artifactId>
			<configuration>
				<skip>true</skip>
			</configuration>
		</plugin>
		<plugin>
			<groupId>org.apache.maven.plugins</groupId>
			<artifactId>maven-jar-plugin</artifactId>
			<configuration>
				<classifier>${android-client-classifier}</classifier>
				<classesDirectory>${project.build.outputDirectory}</classesDirectory>
				<includes>
					<include>${package-client-sources}</include>
				</includes>
			</configuration>
		</plugin>
	</plugins>
</build>
</profile>

Related issues/PRs

Similar issue for Java: #4824
Similar issue for C#: #7875

Suggest a fix/enhancement

// Register our new ByterArray type adapter
gsonBuilder.registerTypeAdapter(byte[].class, new ByteArrayAdapter(client));

[Arodrigal]

I could help fixing and testing this issue.