Apache HTTPClient POSTs to REST service differently than cURL

I am trying to hit a REST API with Apache http client 4.5.5. I can successfully POST to the API using cURL like so:

curl -X POST --user username:password  --header "Content-Type: application/json" --data "@/path/to/file.json" https://some.restfulapi.com/endpoint

However when I try to POST to the API using Apache http client, it always fails with HTTP error code :401 Unauthorized when using the same credentials:

HttpClient httpclient = new DefaultHttpClient();

CredentialsProvider credentialsPovider = new BasicCredentialsProvider();
credentialsPovider.setCredentials(new AuthScope(request.getHost(), 443),  new UsernamePasswordCredentials(user, password));
                

HttpClientContext context = HttpClientContext.create();
context.setCredentialsProvider(credentialsPovider);


HttpPost httppost = new HttpPost(request.getHost()); 


// append headers
for(Header header : request.getHeaders()){
    httppost.addHeader(header.getKey(), header.getValue());
}
                
if(body_entity.length()>0){
    // append the  data to the post
    StringEntity stringentity = new StringEntity(body_entity, HTTP.UTF_8);
    stringentity.setContentType(content_type);
    httppost.setEntity(stringentity);                                           
}


HttpResponse response = httpclient.execute(httppost, context);

I have also tried adding the authentication directly as a header:

String encoding = Base64.getEncoder().encodeToString((user + ":" + password);
httppost.addHeader("Authentication", encoding);

returns a 401 Unauthorized too.

Furthermore, direct header variants:

- httppost.addHeader("user", "Basic " + encoding);
- httppost.addHeader("Authentication", "Basic " + encoding);
- httppost.addHeader(BasicScheme.authenticate(new UsernamePasswordCredentials(user, password), "UTF-8", false));

all result in 400 Bad request response.

Using an HttpClientBuilder with CredentialsProvider

HttpClientBuilder clientbuilder = HttpClients.custom();
clientbuilder = clientbuilder.setDefaultCredentialsProvider(credentialsPovider);
httpclient = clientbuilder.build();

also results in 400 Bad request response.

How do I create an Apache http client POST request that does what the cURL utility is doing? What is cURL doing differently than Apache httpclient? Might the encoding (UTF-8) be the issue?