Doc middleware compress content type

docs/content/middlewares/compress.md

@@ -64,6 +64,9 @@ http:
     * The `Accept-Encoding` request header contains `gzip`.
     * The response is not already compressed, i.e. the `Content-Encoding` response header is not already set.
 
+    If Content-Type header is not defined, or empty, the compress middleware will automatically [detect](https://mimesniff.spec.whatwg.org/) a content type. 
+    It will also set accordingly the `Content-Type` header with the detected MIME type.
+    
 ## Configuration Options
 
 ### `excludedContentTypes`

docs/content/middlewares/contenttype.md

@@ -21,6 +21,9 @@ This middleware exists to enable the correct behavior until at least the default
     is still to automatically set the `Content-Type` header.
     Therefore, given the default value of the `autoDetect` option (false),
     simply enabling this middleware for a router switches the router's behavior.
+    
+    The scope of the Content-Type middleware is the MIME type detection done by the core of Traefik (the server part).
+    Therefore, it has no effect against any other `Content-Type` header modifications (e.g.: in another middleware such as compress).
 
 ## Configuration Examples