There are lots of users who are facing the “Gmail attachment failed” issue. If you also can’t attach files to Gmail then check this article. To avoid problems of attaching files to Gmail, you will need to make some improvements to your Gmail such that the browser should not be in offline mode, check your attachment size limit, check your basic things, clear all the cache and data and update your browser etc. After doing all these fixes you are able to upload attachment files in Gmail.
Also Read - http://www.onlinegeeks.viamagus.com/blog/Why-Can-t-I-Send-Attach-Files-in-Gmail-.html