[BUG][java] Allow requests to return InputStream rather than File

Is your feature request related to a problem? Please describe.

When the Java SDK is paired with the okhttp-gson library requests made with a response body of...

"type": "string",
"format": "binary"

return a File rather than an InputStream or other form of binary data. I am logging the feature request as I've tried to implement this without success and am looking for guidance on the implementation.

Describe the solution you'd like

Requests with a response body of...

"type": "string",
"format": "binary"

return an InputStream rather than relying on a File. Using the useAbstractionForFiles additional property to control this behavior would be acceptable.

Describe alternatives you've considered

I've attempted to implement this functionality within the SDK using a combination of an additional property, useAbstractionForFiles, which already exists and seems to cover this use-case by the description of the property and using Mustache templates.

Additional context

I've come across an issue where in testing {{#returnType}}{{#isFile}}true{{/isFile}}{{/returnType}} within the request never returns true although the return type from the function is of type File.

Start by tracing Java SDK generation for okhttp-gson responses with type: string and format: binary, including the existing useAbstractionForFiles property and Mustache templates. Inspect why {{#returnType}}{{#isFile}} does not evaluate as expected; done means generated requests expose an InputStream or equivalent binary abstraction instead of requiring a File.