Error messages that state that Gmail is unable to attach files may occur if an older version of the Flash plug-in is installed on the computer. The latest version of Flash can be downloaded from the official Adobe website. If Flash is blocked on the computer due to security settings, there may be issues when attaching files in Gmail. The browser should not be in offline mode to avoid problems when attaching files in Gmail. A different browser may be tried after disabling the proxy if Gmail is still unable to attach files.
If you are unable to attach files to Gmail message and you're seeing error messages while uploading attachments, make sure you have Flash installed. Visit Adobe Flash Player to check for updates and download the latest version.
In addition, please make sure that Flash is not being blocked on your computer. If your computer's default browser is Internet Explorer, make sure that it's not in "offline mode."
If you're using Flash 10, try these troubleshooting steps (note that you may not need to do all of the things listed below to fix the problem):