An attachment is a file that you can include as part of your email message. You can attach all types of files to your message, including word processor or spreadsheet documents, audio files, photos, image files (.bmp, .jpg, .gif, etc.), Web pages saved as HTML files, and more. You can attach up to 50 files with a maximum message size of 25 MB.
Here’s How
Begin composing a messages. (Remind me how.)
On the Compose page, click the Attach Files button. (You can attach files at any time while composing a message.)
The Attach Files page opens.
Click the first Browse button.
The Choose File or Open File window opens (depending on your operating system).
Locate the file you want to attach, select it, and click the Open or OK button.
The selected file and its location appear in the first attachment box.
To attach another file, click the next Browse button, and repeat Step 4.
Tips:
You cannot attach the same file multiple times to the same email message.
If you need more attachment boxes, click the Attach More Files link. Yahoo! Mail adds another box.
You can attach one or more files up to a total combined size of 25 MB.
After browsing for all the files you want to attach, click the Attach Files button.
Tip: Norton AntiVirus™ automatically scans all outgoing email attachments and won’t let you attach an infected file.
Progress bars display the process of scanning and attaching each file. When the process is complete, the Compose page opens with the attachments.
Sending Your Message
The order in which you compose your email message is your choice. You can address it, write your message, and then attach files; attach files, write your message, and then address it; or mix it up. Just be sure to have an email address in the To box before you click the Send button.
Source:help.yahoo.com