[BUG] For java Resttemplate Client there is no way to upload file with defined contentType

Hello,
there is no way to do the same request (providing application/pdf) as

curl -X POST -H "content-type: multipart/form-data" -F "file=@pdf-test.pdf;type=application/pdf"

to do this java.io.File should be replaced with more generic org.springframework.http.HttpEntity

    private HttpEntity<Object> createUploadData(MediaType mimeType, InputStream attachment) {
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(mimeType);
        return new HttpEntity<>(new org.springframework.core.io.InputStreamResource(attachment), headers);
    }

HttpEntity allows us to use filename and file mime type.

My proposition is to replace formParams.add("file", new FileSystemResource(file));
with formParams.add("file", httpEntity)

Side notes:

1. java.io.File is very restrictive object type, you can`t extend it in a simple way.
2. selectBody logic is misplaced. by the time we invoke ApiClient.invokeAPI(), MyGeneratedApi shpould determine whether we will use Object body or MultiValueMap<String, Object> formParams to pass form parameters

Start with modules/swagger-codegen/src/main/resources/Java/libraries/resttemplate/api.mustache at the formParams.add("file", ...) call, then review ApiClient.mustache around selectBody and invokeAPI(). Done means generated RestTemplate clients can upload a file with a caller-defined MIME type such as application/pdf.