E-mail Attachment Problems

[#1]

I already have Thunderbird on the old machine - I'd used it for years prior to getting my new one. It's an older version of Thunderbird as well - no update for a year.
I can't send it through that version either - same symptoms.
Weird...especially as both installations worked fine up until a month or so ago.

[#2]

Have you tried a complete re-install of Thunderbird?
You seem to have tried all the options.  If you have a friend with Thunderbird send the email to him via webmail then ask him to try and send it back to you.

[#3]

This is interesting:
I created a new Thunderbird profile on my new machine, set everything up and tried it again.
It still stops at 55%.
Hmmm...

[#4]

I have now found what the problem was...
...it's rather embarrassing, really. 
The Netgear DG834 has a setting which enables you to have Instant Messaging ports open or closed.
I must have closed it, thinking it would increase my security.
Maybe it did - but it also created the problem with e-mail attachments.
Setting it back to 'Open' (the default setting) has cured it. Dunno why this wasn't causing a problem with Outlook, though.
Thanks to all, and apologies for wasting everyone's time. Remember, boys and girls, "When all else fails, read the instructions!"

[#5]

Thanks for reporting back on that one. It's always interesting to learn how things are solved.
Well done on locating the culprit too. I don't think I would ever have tracked that one down.