okhttp-gson and apache-httpclient library (java) cannot upload multipart files with unicode filenames

okhttp-gson and apache-httpclient are the only two (synchronous) libraries which can send multipart files at all.

okhttp-gson fails with the following error message if the file contains unicode characters:

Unexpected char 0xf6 at 34 in Content-Disposition value: form-data; name="file"; filename="öäåß.txt"
java.lang.IllegalArgumentException: Unexpected char 0xf6 at 34 in Content-Disposition value: form-data; name="file"; filename="öäåß.txt"
	at okhttp3.Headers$Companion.checkValue(Headers.kt:434)

apache-httpclient sends ?s instead.

My openapi-generator version is 5.4

This is the relevant part of the openapi declaration:

       "requestBody": {
          "content": {
            "multipart/form-data": {
              "schema": {
                "required": [ "file"  ],
                "type": "object",
                "properties": {
                  "file": {
                    "type": "string",
                    "format": "binary"
                  }

okhttp-gson would work fine if the following lines in ApiClient.java (1293):

                Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"; filename=\"" + file.getName() + "\"");
                MediaType mediaType = MediaType.parse(guessContentTypeFromFile(file));
                mpBuilder.addPart(partHeaders, RequestBody.create(file, mediaType));

are replaced by:

                MediaType mediaType = MediaType.parse(guessContentTypeFromFile(file));
                mpBuilder.addFormDataPart(param.getKey(), file.getName(), RequestBody.create(file, mediaType));

For reference. Here is the Java spring boot code, which tries to accept the requests:

    @Operation(summary = "Upload a file.")
    @PostMapping(value = "functionX", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
    public List<ReturnObject> functionX(@RequestPart("file") MultipartFile excelMultipartFile) throws IOException {

Start with the generated Java ApiClient.java around line 1293 and reproduce the multipart upload using a filename such as öäåß.txt. Compare the okhttp-gson exception and apache-httpclient question marks, then verify the generated clients preserve the Unicode filename and still accept the Spring multipart endpoint shown in the report.