As you attach an important document to an email in Yahoo Mail, a loading-style progress bar and an "Adding an attachment" message help you to visually track the process. A successfully attached document should appear as a file name or image thumbnail at the bottom of the email with links for removing or downloading the attachment as needed. When you don't see these indicators, the process likely failed because of limitations set by Yahoo, browser or computer issues or interruptions on Yahoo's end.
Yahoo Mail doesn't accept attachments with names that contain special characters, such as slashes, hashtags or dollar signs. If the file name contains any symbols or punctuation, remove them and try again. Yahoo also restricts total message size to 25MB. Yahoo estimates that the encoding process it uses to enable your email to be read by other email service providers can increase the size by approximately 33 percent, so your file can't actually be that big. You also can't attach encrypted files, so if you're concerned with keeping prying eyes away, you're out of luck.
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An anti-virus program can be incompatible with Yahoo Mail. Turn your anti-virus software off temporarily and then try attaching the files. If your anti-virus program isn't the problem, turn it back on and scan your system for hidden malware that can cause browser or computer interference. Too many active programs, such as automatic background software updates, can also cause Yahoo Mail errors by putting a strain on system resources. Additionally, old or corrupt data can slow down your computer and browser. Shut down all programs except for your browser, turn off any updates, clear the cache and cookies, restart your computer and then try to attach the files.
Yahoo can experience technical difficulties that prevent you from attaching files even when you can still read, write or edit emails. An upgrade on Yahoo's end can also temporarily shut down certain features. Review third-party sites like Downrightnow and Is It Down Right Now?, tweets on the Yahoo Customer Care Twitter account and comments on the Yahoo Mail Facebook page (link in Resources) for service issue updates. If you don't see your problem, send a public tweet or direct message about it to Yahoo Customer Care on Twitter or add a comment on Facebook below the most recent Yahoo Mail post and then wait for a response.