Yahoo Mail now has Dropbox built in! You can:
To make it even easier, you won't need to sign in when using Dropbox in Yahoo Mail on any computer. This is a special ability we only give to Yahoo, and it requires you to approve “linking” your Dropbox account with your Yahoo account. When you first use Dropbox in Yahoo Mail, you’ll see a screen like the one below that will ask for permission to link your accounts:
By linking your Dropbox and Yahoo accounts, it means you'll automatically get signed into your Dropbox account for use within Yahoo Mail (and nowhere else). Yahoo does not get access to any of the data in your Dropbox unless you specifically pick files to attach to your email.
You can unlink your accounts in the My apps tab of your Dropbox settings. You can also unlink while you're in the Dropbox views inside Yahoo Mail by clicking on your name in the upper right and selecting Sign Out.