Javascript : Validate file upload

Javascript code:

<script language="javascript">
function Checkfiles()
{
var fup = document.getElementById('upload_filename');
var fileName = fup.value;
var ext = fileName.substring(fileName.lastIndexOf('.') + 1);
if(ext == "gif" || ext == "GIF" || ext == "JPEG" || ext == "jpeg" || ext == "jpg" || ext == "JPG" || ext == "doc")
{
return true;
}
else
{
alert("Upload Gif or Jpg images only");
fup.focus();
return false;
}
}
</script>

HTML form:

<form action="#" enctype="multipart/form-data" method="POST" onsubmit="return Checkfiles();">
<input type="file" id="upload_filename">
<input type="submit" value="Go!">
</form>

Or Ruby on Rails form:

<% form_for(@upload, :html => {:multipart => true, :onsubmit=>"return Checkfiles();"}) do |f| %>
<%= f.file_field :filename %>
<%= f.submit 'Create' %>
<% end %>

If you find this useful, would you like to buy me a drink? No matter more or less, it will be an encouragement for me to go further. Thanks in advance!! =)