mail attachments?

That's on a web mail server.

You absolutely need to wait untill your attachment have been completely uploaded to the server BEFORE you click on the Send button.
If the file fails to upload, cancell the transfert and start again.
If the file is not already compressed, compress it as a zip, rar, tar or some other compressed archive format.

Make sure that your attachment don't exceed the maximum allowed size.

Try uploading your attachment to some file exchange service, and send a link to that remote copy of your file.

For ADSL connections, the upload speed is slower than the download speed, sometimes, MUCH slower...
If you still use dialup, the upload speed is probably no more than half your download speed.

Electro
August 2013