Cannot add attachment to custom list form in SharePoint 2007

The problem is that whenever you customize a list form—for example, remove assignment and status fields from a form to submit new issues—you break the ability for the user to add attachments to the list item from that form. In most cases, we got an "Error on Page" at in the status bar with the following message shown in the Error Details:

Can't move focus to the control because it is invisible, not enabled, or of a type that does not accept the focus.

We finally have a resolution now, although it does take code modifications to each customized list form. See the complete details here:

http://support.microsoft.com/kb/953271

**Pay careful attention to the code. For new item forms, there are SEVERAL changes required to the code and the changes must be made in multiple places.

Source: https://staceybailey.securespsites.com/blog/Lists/Posts/Post.aspx?ID=13