You have to make a form post to get this to work; you can't just make an HTTP GET passing the file name in the query string.

To illustrate, here's how you'd do it in curl (the options are put on different lines for readability):

curl --form feedItemFileUpload=@/home/me/test.txt
     --form fileName=contentFile
     --form "desc=This is a test file that I'm posting." 
     --form "text=I love posting files to Chatter!" 
     --header 'Authorization: OAuth 00Dx0000000X42V!AQgAQClp6UqGWNJUf4d3LQkVCysBIS6afWAGodgODJ.o1XQkE_fAnZIv5gzfRXxSf_D.28fsauDdZVE1nQU9lMqQSfOJTtb3' 
     https://na1.salesforce.com/services/data/v22.0/chatter/feeds/user-profile/me/feed-items

And here's how you'd do it in Java:

import java.io.File;

import org.apache.commons.httpclient.*;
import org.apache.commons.httpclient.methods.PostMethod;
import org.apache.commons.httpclient.methods.multipart.*;

...

String oauthToken = "00Dx0000000X42V!AQgAQClp6UqGWNJUf4d3LQkVCysBIS6afWAGodgODJ.o1XQkE_fAnZIv5gzfRXxSf_D.28fsauDdZVE1nQU9lMqQSfOJTtb3";
String url = "https://na1.salesforce.com/services/data/v22.0/chatter/feeds/user-profile/me/feed-items";
String text = "I love posting files to Chatter!";
File contentFile = getFile();
String desc = "This is a test file that I'm posting.";
String fileName = "contentFile";
Part[] parts = {
        new StringPart("desc", desc),
        new StringPart("fileName", fileName),
        new StringPart("text", text),
        new FilePart("feedItemFileUpload", contentFile),
};

final PostMethod postMethod = new PostMethod(url);
                
try {
    postMethod.setRequestEntity(new MultipartRequestEntity(parts, postMethod.getParams()));
    postMethod.setRequestHeader("Authorization", "OAuth " + oauthToken);
    postMethod.addRequestHeader("X-PrettyPrint", "1");
    HttpClient httpClient = new HttpClient();
    httpClient.getParams().setSoTimeout(60000);
    int returnCode = httpClient.executeMethod(postMethod);
    System.out.println(postMethod.getResponseBodyAsString());
    assertTrue("Expected return code of: " + HttpStatus.SC_CREATED, returnCode == HttpStatus.SC_CREATED);
} finally {
    postMethod.releaseConnection();
}