In most cases where this occurs, it is caused by an incorrect URL that has been linked to in the email.  We most often find that this specific issue is caused when an errant character or a space is put into a URL or original filename during the linking process.  

To resolve the issue, add files or images with no spaces in the file name.  For more information about using spaces in filenames, click here

If the file has no spaces and the link is formatted correctly, Chat with Support and reference this article. .