For _APIs_ I prefer to use both - the only downside is that resource names need to be restricted to _not_ include trailing `.{EXT}`s (either at all or limiting

For _APIs_ I prefer to use both - the only downside is that resource names need to be restricted to _not_ include trailing `.{EXT}`s (either at all or limiting EXT to things that aren't valid content types). E. g. `/books` - looks at the `Accept` header. `/books.json` - sets the `Accept` header to `application/json`. `/books.xml` - `application/xml`, and so on.