Assign multiple categories to Media Library upload

Here’s how I did it, using a form to id file and set doc type, uploading with media_handle_upload, then post-processing using the attachmentID and combo box choice from the form with wp_set_object_terms:

  function myFileUploader() {
        if (isset($_POST['submit'])) {
            require_once( ABSPATH . 'wp-admin/includes/image.php' );
            require_once( ABSPATH . 'wp-admin/includes/file.php' );
            require_once( ABSPATH . 'wp-admin/includes/media.php' );
            $doctype = $_POST['doctype'];
            $attachment_id = media_handle_upload("fileToUpload", 0); 
                if (is_wp_error($attachment_id)) {
                    // There was an error uploading the image.
                    echo "Error adding file";
                } else {
                    // The file was uploaded successfully!
                    echo "AttachmentID  is " . $attachment_id  . "<br>";
                    execute_on_add_attachment_event($attachment_id, $doctype);
                } 
      }
      echo '
        <form action="" method="post" enctype="multipart/form-data">
          <input type="file" name="fileToUpload" id="fileToUpload">
          <label for="doctype">Choose a document type:</label>
          <select id="doctype" name="doctype">
            <option value="Newsletter">newsletter</option>
            <option value="Minutes">minutes</option>
            <option value="Treasurer Report">treasurer</option>
          </select>
          <input type="submit" value="Upload File" name="submit"/>
        </form>
      ';
    }
    
    function myFileUploaderRenderer() {
      ob_start();
      myFileUploader();
      return ob_get_clean();
    }
        
    add_shortcode('custom_file_uploader', 'myFileUploaderRenderer');

2. Post-processor:

    function execute_on_add_attachment_event($attachment_id, $doctype){
    switch ($doctype) {
      case 'Minutes':
            $uploadCategory = array(946,974,933,923);
            break;
      case 'Treasurer Report':
            $uploadCategory = array(946,935);
            break;
      case 'Newsletter':
            $uploadCategory = array(946,975,922,937);
            break;
      default:
            $uploadCategory = array(946);
            break;
    }
    wp_set_object_terms( $attachment_id, $uploadCategory, 'category' );
    }