Issue with file uploads

Hi all;

I am trying to add a file submission aspect to a submission form, but I cannot get uploads to confirm they are uploaded:

---------------FORM-----------------------

<input type="hidden" name="MAX_FILE_SIZE" value="2048">
<input name="File" type="file" size="50">

----------------/FORM----------------------

---------------CODE------------------------

 if (isset($_FILES['File']['name']))
                        {
                                echo("File Uploaded\n");
                                print_r($_FILES['File']);
                        }
                        else
                        {
                                echo("no File Uploaded\n");
                                print_r($_FILES);
                        }

---------------/CODE---------------------------

-------------OUTPUT---------------------------
no File Uploaded
Array ( )
--------------/OUTPUT------------------------

Now I am new to file uploading, so maybe I am checking the wrong thing.
PHP is 4.2.2 on Apache 2, I will also need to check that the file is a zip file, not interested in uploads of anything else and will want to reject non-zip uploads.

Thanks all, big points cause I want a quick answer.