Added ext prefix to both link insertion scenarios

Previous commit didn't add the `file-` prefix on both link insertion scenarios (i.e. selecting text to turn into a file link, and inserted a new file link using its title). 

Also, I've altered it to include a separate class `file`, and changed the prefix to `ext-`. This is to allow standard pseudo-element styles that will be applied to all such links to be included in the `file` class (e.g. `:content { display: inline-block; font-family: FontAwesome; font-size: .8em }` etc) and individual extension-specific classes to be defined on the basis of file types (e.g. `.file.ext-pdf:before { content: "\f1c1" }`). This results in smaller and more efficient CSS because the `file` stuff isn't being repeated.
Merge pull request #1855 from melat0nin/patch-7

Added ext prefix to both link insertion scenarios