I have a similar problem. Only sometimes, when I attach files and make sure they have uploaded then "send", the recipient replies there is no attachment. I however see that in "Sent Items" there is no attachment, indicated by a paperclip, when this happens.
First you doubt yourself because it seems an intermittent fault, now I think I have found a trend.
When i send more than about 10 attachments across 1 or more emails it loses the attachments.
It may be an accumulate size or number of emails that causes the lost attachments, I have not checked up on that.
If I restart the email session it seems to work again until I get up to 10 or so.
Hope this information and workaround helps. Would be much better if it worked.