1. Sign in to Yahoo Mail
2. Click on the attachment.
3. Click on the 'Save to DropBox' button
4. You're done
If you don not have a DropBox account as yet, you can click on 'More Options' to create a new account and name a new folder to save all your attachments.
How to share a file or folder from DropBox in Yahoo mail
1. Click on Compose.
2. Next to the paper clip icon select the drop-down button
3. Select 'Share from DropBox'. The chooser will open and you can then select files you wish to share. If the attachments is large you can simply drag and drop the file into email and click 'Send' to send from DropBox. This integration is available globally.
Source: Yahoo Blog and DropBox Blog.