It's commonplace for security software to block, rename or remove files attached to e-mails. That's because so many junk e-mails have viruses attached. The usual way round this is, as Powertool has suggested, to compress the file. You can do this with Winzip (shareware) or Zipcentral (freeware). Winzip will also allow you to split an attachment and spread it over multiple e-mails to get around size limits.
Try sending yourself an e-mail with the file attached. This works just like posting yourself a letter. If it arrives you'll know that neither your PC nor your own ISP is blocking the attachment. If it doesn't work, try sending yourself the zipped file. Once you know you can get it out onto the internet you can try sending it to the intended recipient.