What do you want to achieve by "blocking" them?

You don't have to open them, so why care if they are there?

Or are you anxious about bandwidth or storage capacity?

My e-mails open with attachments automatically.

What sort of attachments are they? Pictures and text files, yes, they would display automatically.

But if the message is not from someone you know or trust, why open it?

The simple truth is that "blocking" attachments isn't easy. Everything that comes with an email message is usually encoded into the message. The interpretation of part of the message as an attachment is really a matter of semantics. It's not easy for a mail client to identify an attachment as such until it has actually downloaded and parsed the message.

If you don't want to see attachments, I'd set these options in Thunderbird:

View|Message Body As|Plain Text View|Display Attachments Inline unticked