Clearing your Outlook attachment cache can solve problems with opening or downloading attachments in Outlook. This cache is made up of previously downloaded attachments, but the folder can grow too large or become corrupt. This will sometimes cause errors. To clear this cache, navigate to the following place on your computer:
C:\Users\<username>\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.Outlook
Delete all files and folders inside of Content.Outlook. This removes all your previously downloaded attachments.