[BUG] Uploading large file from client code as multipart/form-data leads to high memory usage

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Hello,
We're observing OutOfMemoryErrors when uploading a large file from a client application as multipart/form-data. It appears the complete file is read into memory, however, I think this should be done streamed. Uploading a ~300MB file seems to use ~1000MB of heap memory.

Stacktrace
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space at java.base/java.lang.StringCoding.decodeUTF8_0(StringCoding.java:777) at java.base/java.lang.StringCoding.decodeUTF8(StringCoding.java:734) at java.base/java.lang.StringCoding.decode(StringCoding.java:257) at java.base/java.lang.String.<init>(String.java:507) at java.base/java.lang.String.<init>(String.java:561) at okio.Buffer.readString(Buffer.kt:306) at okio.Buffer.readUtf8(Buffer.kt:290) at foo.bar.baz.ApiClient.requestBodyToString(ApiClient.java:1558) at foo.bar.baz.ApiClient.buildRequest(ApiClient.java:1209) at foo.bar.baz.ApiClient.buildCall(ApiClient.java:1157)

openapi-generator version

6.4.0

OpenAPI declaration file content or url

OpenAPI specification (simplified)

paths:
  /upload:
    post:
      tags:
      - foo
      summary: bar
      description: |
        baz
      requestBody:
        content:
          multipart/form-data:
            schema:
              body:
                 properties:
                    file:
                      type: string
                      format: binary
        required: true

Generation Details

Using Java generator with Maven.

Steps to reproduce

• generate client code
• start application with limited amount of heap memory
• call client code with significantly large file

Related issues/PRs

Suggest a fix

Start with the generated ApiClient.java methods requestBodyToString, buildRequest, and buildCall named in the stack trace, then trace how the Java generator handles multipart/form-data file uploads. Reproduce the upload with a roughly 300 MB file under a limited heap; done means the client uploads it without converting the complete file into an in-memory string or raising OutOfMemoryError.