Improve error messages for inserting local files which aren't images

[clarkepaul]

When trying to insert .doc or .mp3 onto a page you get the message "Embed is only compatible with remote files", this is currently true but we can suggest they link to the file instead. Instead of showing this error message we should be able to detect the type of file that is selected and explain before they click the insert button that this file can't be inserted directly. We even hide them in the insert modal but that could be confusing not showing all files.

It would be good to allow inserting other media directly into a page sometime in the future but can understand it wouldn't be very high priority.