I'm using the code below to create a feed element with an attachment, using Chatter REST API, v32.0.

        File feedElementDefinition = new File("feed3.txt");
        String json = IOUtils.toString(new FileInputStream(feedElementDefinition));

        CloseableHttpClient client = HttpClients.createDefault();
        HttpPost post = new HttpPost(
            "https://eu3.salesforce.com/services/data/v32.0/chatter/feed-elements" +
            "?feedElementType=FeedItem" +
            "&subjectId=<id of my user account>" +
            "&text=Text"
        );
        post.addHeader(HttpHeaders.AUTHORIZATION, token());
        post.addHeader(HttpHeaders.CONTENT_TYPE, ContentType.MULTIPART_FORM_DATA.getMimeType());
        post.addHeader(HttpHeaders.ACCEPT, ContentType.WILDCARD.getMimeType());
        post.setEntity(
            MultipartEntityBuilder.create()
                .addBinaryBody(
                    "feedElementFileUpload",
                    new File("feed_element.txt"),
                    ContentType.APPLICATION_OCTET_STREAM,
                    "feed_element.txt"
                )
                .addTextBody(
                    "json",
                    json,
                    ContentType.APPLICATION_JSON
                ).build()
        );
        CloseableHttpResponse response = client.execute(post);

Contents of `feed3.txt` are:

    {
      "body" : {
        "messageSegments" : [ {
          "text" : "Some dummy status",
          "type" : "Text"
        } ]
      },
      "capabilities" : {
        "associatedActions" : null,
        "bookmarks" : {
          "isBookmarkedByCurrentUser" : false
        },
        "content" : {
          "description" : "feed_file",
          "title" : "feed3.txt"
        }
      },  
      "feedElementType" : "FeedItem",
      "originalFeedElementId" : null,
      "visibility" : "AllUsers",
      "subjectId" : "<my user account id>"
    }

However, it fails with the following message:

    You have sent us an Illegal URL or an improperly formatted request.

What's the reason for this? I'm going almost crazy on this one, since when I use cURL (show below), it's fine.

    curl -H "X-PrettyPrint: 1" -H "Content-Type: multipart/form-data" -F 'json={ "body" : { "messageSegments" : [ { "text" : "Some dummy status", "type" : "Text" } ] }, "capabilities" : { "associatedActions" : null, "bookmarks" : { "isBookmarkedByCurrentUser" : false }, "content" : { "description" : "feed_file", "title" : "feed3" } },  "feedElementType" : "FeedItem", "originalFeedElementId" : null, "visibility" : "AllUsers", "subjectId" : "my_user_account_id" };type=application/json' -F "feedElementFileUpload=@/home/haris/projects/open-source/java-playground/feed3.txt;type=application/octet-stream" 
    -X POST 'https://eu3.salesforce.com/services/data/v32.0/chatter/feed-elements?feedElementType=FeedItem&subjectId=my_user_account_id&text=Text' -H 'Authorization: OAuth my_token_blah_blah' --insecure

Thanks in advance!