Unfortunately, your account is experiencing a temporary issue with Yahoo! Mail’s attachment feature, which can affect either incoming attachments or outgoing attachments, or both. That is why it may be affecting one, some, or all of your accounts with no sense of logic behind it, and why the same problem is happening on more than one computer.
Some users are reporting success with an alternative web browser, such as Firefox. You can download this faster, safer, more secure web browser from here--> http://www.firefox.com
Some users are also reporting that going back to Yahoo! Mail Classic temporarily helps with the problem. Here is the link to Yahoo! Mail Classic --> http://us.mc330.mail.yahoo.com/mc/welcome?ymv=0 (You can press CTRL + D to bookmark that page.)
If neither of those works, you will need to wait 12-48 (and in some rare cases, up to 72) hours for that functionality to be fully restored.
If you need to send email right away with attachments, you may consider uploading the file(s) to http://www.wikisend.com/ and then after you’ve uploaded it, provide the file link to the recipient so that they can download it from there.
Please Reply with additional questions as I am here to help you with any additional concerns you may have. Thank you.