I have the following configuration for nginx in AWS source bundle.

location / {
    proxy_pass http://127.0.0.1:5000;

    proxy_http_version  1.1;

    proxy_set_header    Connection          $connection_upgrade;
    proxy_set_header    Upgrade             $http_upgrade;
    proxy_set_header    Host                $host;
    proxy_set_header    X-Real-IP           $remote_addr;
    proxy_set_header    X-Forwarded-For     $proxy_add_x_forwarded_for;
}

Controller as below.

@GetMapping("/inputuploads") 
public String listInputUploads(Model model) {       
    return "inputuploads";
}

@PostMapping("/inputuploads")
public String saveFile(Model model, @RequestParam("file") MultipartFile file, RedirectAttributes ra) throws IOException {
    s3Service.uploadFile(file.getOriginalFilename().toLowerCase(), file);
    ra.addFlashAttribute("message","Uploaded file: "+ file.getOriginalFilename());
    return "redirect:/inputuploads";
}

Template as below.

            <div >
                <form method="post" th:action="@{/inputuploads}" enctype="multipart/form-data" >                                    
                    <div>
                        <input type="file" name="file" accept=".xlsm" >                                          
                    </div>                    
                    <div>
                        <button type="submit" >Upload</button>
                    </div>                    
                </form>                
            </div>

The GET url is accessed through https and while submitting POST, after saving the file in S3, it is redirecting to http url instead of https.