Why won't my attachments attach? 

If you can't attach files to Outlook emails, the problem may be with your computer's file system. The email will not be able to open the attachments if the file is bigger than 20MB. To fix this, you should try to download and install the latest version of your operating system. This should prevent the problem from occurring again in the future. If you're using Mac OS, then it's best to update your macOS to the latest version.