Sending Attachments in Yahoo Mail
Sometimes things don't work the way we want them to work.
Or wait, maybe they do, we just can't figure it out!
In this instance a user wanted to send an attachment using web-based Yahoo Mail. When the user right-clicked on the desired file and chose 'Send to/Mail Recipient', it brought up an un-used email client with no email set up to send or receive email.
The user didn't realize there is a way to send attachments right from Yahoo Mail.
While in Yahoo Mail, click New to start a new message and click the Attach button at the top (next to the Send button). A dialog box will open to setup the 'Easy Attach tool.
(Click to enlarge)
Tick the box to agree to the terms of service. 
Now click New for new message. Add your recipient and subject line message and click 'Attach'. A browse window will open to your Documents and you can select the file you want to send. Now you can see your attachment is at the bottom of your message.
(Click to enlarge)