I have a feeling that we are dealing with a Lotus Notes client application here. In such an environment, I find it difficult to do what you want to do out of the box.
You could use the Midas Rich Text add-on for Lotus Notes Domino (made by geniisoft.com -- and is highly recommended). I am sure Midas would make it possible to hide rich-text programmatically for you. I would suggest it especially if the attachment has to be in the Lotus Notes rich-text field.
To programmatically hide and/or show an attachment in a Lotus Notes rich-text field, you need only place the rich-text field in a "Section," with a hide-when formula. That's it! No third party add-on is required.
Do you have comments on this Ask the Expert Q&A? Let us know.
Related information from SearchDomino.com:
Dig Deeper on Web Development for Lotus Notes Domino
Related Q&A from Jens Bruntt
Learn about an add-on Domino developers can use to programmatically edit rich-text field tables from within the Lotus Notes client. Continue Reading
Web Development expert Jens Bruntt outlines three potential issues to consider when troubleshooting Lotus Notes Domino tabbed table problems. Continue Reading
Learn two Lotus Notes Domino commands you can use to verify if users have access to Web pages from within Lotus Notes. Continue Reading