This appears if the Safari cache file is either deactivated or corrupt.
If you deactivated the cache, then you'll need to use another browser, such as Firefox, for your attachments and uploads.
Otherwise, a corrupt cache can be rectified by moving the existing folder (User Library>Caches>Safari)to the trash and restarting Safari. Safari automatically creates a new cache folder.
Please consider marking this post "solved" to the right of my name (you'll need to log-on to the forum). Helpful for others looking for a solved solution to a similar question, also helpful for me.