One of Yosemite's nifty new features is a little something called Mail Drop, which automatically uploads large email attachments to iCloud and allows people to download them when they receive the email. Perfect for mail servers with size limits.
It's enabled by default for iCloud mail, but for other accounts, you'll have to switch it on. Here's how:
Open Mail > Preferences > Accounts
Select the account you want to enable Mail Drop for
Click the Advanced tab
Tick the box labeled Send large attachments with Mail Drop
Easy as pie! Now go and find a huge attachment so you can test it out.
Sing your song in the comments.