[SalesForce] Why would this Chatter REST Api Http POST return a 500 Server Error

I was able to copy and execute your code as standalone java project, here is the code it works like charm and creates the chatter post with image and text. Seems this is because of the way HTTP request is composed by SalesforceRestClient. Try sending the post request using HTTP Client lib directly i.e. without SalesforceRestClient, it might work

import java.io.File;
import java.nio.charset.Charset;

import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.mime.HttpMultipartMode;
import org.apache.http.entity.mime.MultipartEntity;
import org.apache.http.entity.mime.content.FileBody;
import org.apache.http.entity.mime.content.StringBody;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;

public class Test {
    // TODO:change node from ap1 to your node
    private static final String RES_URL = "https://ap1.salesforce.com/services/data/v25.0/chatter/feeds/news/me/feed-items";
    // TODO:change this before run
    private static final String OAUTH_TOKEN = "OAuth <YOUR OATUH TOKEN GOES HERE>";

    public static void main(String[] args) throws Exception {
        HttpClient httpclient = new DefaultHttpClient();
        HttpPost httppost;
        MultipartEntity reqEntity;
        httppost = new HttpPost(RES_URL);
        reqEntity = new MultipartEntity(HttpMultipartMode.BROWSER_COMPATIBLE);
        File imageFile = new File(
                "/Users/abhinav/Downloads/chatter-rest-test.jpeg");
        FileBody bin = new FileBody(imageFile);
        reqEntity.addPart("feedItemFileUpload", bin);
        String fileName = "chatter-rest-test.jpeg";
        // file name can be text plain only, though using text/html doesn't breaks
        reqEntity.addPart("fileName", new StringBody(fileName, "text/plain",
                Charset.defaultCharset()));
        // Sending text/html doesn't helps as HTML will be printed, though using text/html doesn't breaks
        reqEntity.addPart("text", new StringBody("Hellow World", "text/plain",
                Charset.defaultCharset()));
        reqEntity.addPart("feedItemFileUpload", new FileBody(imageFile,
                fileName, "application/octet-stream", Charset.defaultCharset()
                        .toString()));
        httppost.setEntity(reqEntity);

        httppost.setHeader("Authorization", OAUTH_TOKEN);

        String response = EntityUtils.toString(httpclient.execute(httppost)
                .getEntity(), "UTF-8");

        System.out.println(response);
        // Time to see the glorious post in web browser
    }
}

Here's the POST data that ended up working for me. I had to use JSON instead of XML in the first part of the POST (after the first boundary) because XML was returning an UNKNOWN EXCEPTION. I'm sure there's a way to use XML, though; I just don't know how.

POST https://cs7.salesforce.com/services/data/v26.0/chatter/users/[my-user-id]/photo HTTP/1.1
Accept: application/xml
Content-Type: multipart/form-data; boundary=8cfa66b1737e032
Authorization: OAuth [my-session-id]
Host: [my-Salesforce-instance-name].salesforce.com
Content-Length: 32075
Expect: 100-continue

--8cfa66b1737e032
Content-Disposition: form-data; name="json"
Content-Type: application/json; charset=UTF-8

{}
--8cfa66b1737e032
Content-Disposition: form-data; name="fileUpload"; filename="photo.jpg"
Content-Type: application/octet-stream; charset=ISO-8859-1
Content-Transfer-Encoding: binary

[binary-file-bytes]
--8cfa66b1737e032--