WIP: Add mime-ext-go to support more MIME-Types

@nick.thomas Wouldn't that be enough ?

After go get it should fetch the mime-ext-go project which just loops through this 1.000 extensions list and adds each MIME-Types using mime.AddExtensionType(extension, MimeType) on init().

If you prefer a standalone implementation of it without the dependency, just let me know. Were you thinking into some other solution ?