Hello,
To attach more than one file to an email in the new Yahoo Mail you need to click the paper clip icon (circled in red) for each file that you want to attach:

When you are done you can click the carrot (circled in blue) to confirm that all of your files attached successfully.
Did you know that Yahoo offers versions of Internet Explorer and Mozilla Firefox that are optimized for Yahoo Mail?
To attach more than one file to an email in the new Yahoo Mail you need to click the paper clip icon (circled in red) for each file that you want to attach:

When you are done you can click the carrot (circled in blue) to confirm that all of your files attached successfully.
Did you know that Yahoo offers versions of Internet Explorer and Mozilla Firefox that are optimized for Yahoo Mail?