Siena (and most places) sets a limit to how large a file you can send via e-mail. This makes sense: large file attachments take up a lot of space on a server. Most mail messages are only a few kilobites; thus a 1 Meg attachment uses up the space of hundreds of regular e-mail messages.
So how do you send a large file? Well, the techie answer is that e-mail isn't designed for file transfers and you should just put the file up on an FTP site and have people download it from there.
Not very helpful; most people don't even know what that means. Luckily there's a simple solution (actually, there are several, but this is the site I use): Mailbigfile.com
It works fairly simply: Go to the site, enter the e-mail address off the recipient, your e-mail address, and any message and then upload the file (up to 100 Meg). An e-mail will be sent with a link that will allow the recipient to download the file. It will be available for a week, then be deleted.
I've found this to be a very useful tool to get around attachment restrictions.