Android : Uploading file to remote server from android isn't working

I am a new Android learner. I was trying to upload a textfile from SD card to a remote server. I have written the PHP script in such a way that If the file is set it will do file operations and response "Success!" else it will response "File is not set!". Here is the PHP Script -

<?php

    if(isset($_FILES["file"])){
        /* Do file operations */
        echo "Success!";
    }
    else{
        die("file is not set!");
    }

?>

It always responses "file is not set!". May be its not recognizing the "file" index in $_FILES[]. Can't understand why!

Here is the android part for uploading file:

/* Location of the file */
String filePath = Environment.getExternalStorageDirectory() + "/sample.txt";

/* URL of the PHP script for file processing */
String URL = "http://myserver/upload.php";

/* New HTTP Client */
HttpClient client = new DefaultHttpClient();

/* Post */
HttpPost post = new HttpPost(URL);

/* Create file from file path */
File file = new File(filePath);
FileBody fileBody = new FileBody(file);

/* Make MultipartEntity from File Body and send */
MultipartEntity reqEntity = new MultipartEntity(HttpMultipartMode.BROWSER_COMPATIBLE);
reqEntity.addPart("file", fileBody);

/* Execute Http post request and get response */
HttpResponse response = client.execute(post);
HttpEntity resEntity = response.getEntity();

if(resEntity != null){
    /* Make response entity into string to get string response */
    String responseString = EntityUtils.toString(resEntity);
    Toast.makeText(getApplicationContext(), responseString, Toast.LENGTH_SHORT).show();
}

And here is full PHP part:

<?php

    if(isset($_FILES["file"])){
        $file_path = "upload/" . basename($_FILES["file"]["name"]);

        if(move_uploaded_file($_FILES["file"]["tmp_name"], $file_path)){
            echo "Success!";
        }
        else{
            echo "Failed!";
        }
    }
    else{
        die("file is not set!");
    }

?>