[BUG] [JAVA] Passed in Content-Type is overwritten as `application/json`

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?

Description

We have a spec where one of the request bodies allows for multiple content-types. If a user specifies a particular content type, say image/jpeg, the generated SDK will be default overwrite that content type always to application/json with a selectHeaderContentType function defined below. This should be changed so that passed in Content-Types are not overwritten by this generated function. I couldn't figure out any way to override this with mustache templates since I had no way to pull in the content-type type defined by the user when using the SDK

    /**
     * Select the Content-Type header's value from the given array:
     *   if JSON exists in the given array, use it;
     *   otherwise use the first one of the array.
     *
     * @param contentTypes The Content-Type array to select from
     * @return The Content-Type header to use. If the given array is empty,
     *   returns null. If it matches "any", JSON will be used.
     */
    public String selectHeaderContentType(String[] contentTypes) {
        if (contentTypes.length == 0) {
            return null;
        }

        if (contentTypes[0].equals("*/*")) {
            return "application/json";
        }

        for (String contentType : contentTypes) {
            if (isJsonMime(contentType)) {
                return contentType;
            }
        }

        return contentTypes[0];
    }

openapi-generator version

OpenAPI declaration file content or url

Here is a minimal version that just includes the part that's breaking

openapi: 3.0.3
info:
  title: Messaging
  version: 4.3.0
  contact:
    name: Bandwidth
    url: https://support.bandwidth.com
    email: support@bandwidth.com
  description: |-
    The API Specification for Bandwidth's Messaging Platform

    ## Base URL

    `https://messaging.bandwidth.com/api/v2`
servers:
  - url: https://messaging.bandwidth.com/api/v2
    description: Production
paths:
  /users/{accountId}/media:
    put:
      summary: Upload Media
      description: |-
        Upload a file. You may add headers to the request in order to provide some control to your media file.
      operationId: uploadMedia
      tags:
        - Media
      parameters:
        - $ref: "#/components/parameters/accountId"
        - $ref: "#/components/parameters/mediaId"
        - $ref: "#/components/parameters/contentType"
        - $ref: "#/components/parameters/cacheControl"
      requestBody:
        $ref: "#/components/requestBodies/uploadMediaRequest"
      responses:
        "204":
          description: No Content
components:
  parameters:
    accountId:
      in: path
      name: accountId
      required: true
      schema:
        type: string
      description: Your Bandwidth Account ID.
      example: "9900000"
    mediaId:
      in: path
      name: mediaId
      required: true
      description: Media ID to retrieve.
      example: 14762070468292kw2fuqty55yp2b2/0/bw.png
      schema:
        type: string
    contentType:
      in: header
      name: Content-Type
      style: simple
      explode: false
      description: The media type of the entity-body.
      example: audio/wav
      schema:
        type: string
    cacheControl:
      in: header
      name: Cache-Control
      style: simple
      explode: false
      description: >-
        General-header field is used to specify directives that MUST be obeyed by
        all caching mechanisms along the request/response chain.
      example: no-cache
      schema:
        type: string
  schemas:
  requestBodies:
    uploadMediaRequest:
      content:
        application/json:
          schema:
            type: string
            format: binary
        application/ogg:
          schema:
            type: string
            format: binary
        application/pdf:
          schema:
            type: string
            format: binary
        application/rtf:
          schema:
            type: string
            format: binary
        application/zip:
          schema:
            type: string
            format: binary
        application/x-tar:
          schema:
            type: string
            format: binary
        application/xml:
          schema:
            type: string
            format: binary
        application/gzip:
          schema:
            type: string
            format: binary
        application/x-bzip2:
          schema:
            type: string
            format: binary
        application/x-gzip:
          schema:
            type: string
            format: binary
        application/smil:
          schema:
            type: string
            format: binary
        application/javascript:
          schema:
            type: string
            format: binary
        audio/mp4:
          schema:
            type: string
            format: binary
        audio/mpeg:
          schema:
            type: string
            format: binary
        audio/ogg:
          schema:
            type: string
            format: binary
        audio/flac:
          schema:
            type: string
            format: binary
        audio/webm:
          schema:
            type: string
            format: binary
        audio/wav:
          schema:
            type: string
            format: binary
        audio/amr:
          schema:
            type: string
            format: binary
        audio/3gpp:
          schema:
            type: string
            format: binary
        image/bmp:
          schema:
            type: string
            format: binary
        image/gif:
          schema:
            type: string
            format: binary
        image/jpeg:
          schema:
            type: string
            format: binary
        image/pjpeg:
          schema:
            type: string
            format: binary
        image/png:
          schema:
            type: string
            format: binary
        image/svg+xml:
          schema:
            type: string
            format: binary
        image/tiff:
          schema:
            type: string
            format: binary
        image/webp:
          schema:
            type: string
            format: binary
        image/x-icon:
          schema:
            type: string
            format: binary
        text/css:
          schema:
            type: string
            format: binary
        text/csv:
          schema:
            type: string
            format: binary
        text/calendar:
          schema:
            type: string
            format: binary
        text/plain:
          schema:
            type: string
            format: binary
        text/javascript:
          schema:
            type: string
            format: binary
        text/vcard:
          schema:
            type: string
            format: binary
        text/vnd.wap.wml:
          schema:
            type: string
            format: binary
        text/xml:
          schema:
            type: string
            format: binary
        video/avi:
          schema:
            type: string
            format: binary
        video/mp4:
          schema:
            type: string
            format: binary
        video/mpeg:
          schema:
            type: string
            format: binary
        video/ogg:
          schema:
            type: string
            format: binary
        video/quicktime:
          schema:
            type: string
            format: binary
        video/webm:
          schema:
            type: string
            format: binary
        video/x-ms-wmv:
          schema:
            type: string
            format: binary
      required: true
  securitySchemes:
    Basic:
      type: http
      scheme: basic
      description: |-
        Basic authentication is a simple authentication scheme built into the
        HTTP protocol. To use it, send your HTTP requests with an Authorization
        header that contains the word Basic followed by a space and a
        base64-encoded string `username:password`Example: `Authorization: Basic
        ZGVtbZpwQDU1dzByZA==`
security:
  - Basic: []
tags:
  - name: Media

Steps to reproduce

Reproducible by using the generated SDK and designating a specific content-type in the UploadMedia call. If you check what content-type header is actually being sent out it is different from what was specified by the user

Start by tracing the generated Java SDK's selectHeaderContentType function and the UploadMedia call in the reproduced SDK. Check how the user-supplied Content-Type reaches header selection and add or update coverage for a non-JSON type such as image/jpeg. Done means an explicitly passed Content-Type is preserved rather than replaced with application/json.