It's a bit convoluted.. The Attachment tab on MyForums gives a list of old attachments.

http://forums.delphiforums.com/myforums

Click on any item on the list to go to the original message and Edit the message to get a link to remove attachments. The edit screen will show the attachment at the bottom in Zeta to be xed out, or at the top in Classic with a pencil to the right to remove it.

However ...

When it's images you are attaching, it may be better to use Insert Image on the editor toolbar to add images directly into messages. There is no limit that way and it looks better:

http://www.delphiforums.com/howto/insertImage.htm

let us know how it goes  ...