clarify Content-Type / Content-Encoding / Content-Language handling

closes tus#158

protocol.md

@@ -45,6 +45,7 @@ Kleidl](https://twitter.com/Acconut_)<br>
 [J. Ryan Stinnett](https://convolv.es),
 [Ifedapo Olarewaju](https://github.com/ifedapoolarewaju)
 [Robert Nagy](https://github.com/ronag)
+[Nils Goroll](https://github.com/nigoroll)
 
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
 "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
@@ -299,7 +300,7 @@ Host: tus.example.org
 Content-Length: 0
 Upload-Length: 100
 Tus-Resumable: 1.0.0
-Upload-Metadata: filename d29ybGRfZG9taW5hdGlvbl9wbGFuLnBkZg==,is_confidential
+Upload-Metadata: filename d29ybGRfZG9taW5hdGlvbl9wbGFuLnBkZg==,filetype YXBwbGljYXRpb24vcGRm,is_confidential
 ```
 
 **Response:**
@@ -334,6 +335,34 @@ Since metadata can contain arbitrary binary values, Servers SHOULD
 carefully validate metadata values or sanitize them before using them
 as header values to avoid header smuggling.
 
+Clients and servers SHOULD implement the metadata key ``filetype``
+with the value containing the actual ``Content-Type`` of the Upload
+(because the ``Content-Type`` header is formally required to be set to
+``application/offset+octet-stream`` for [Creation with
+Upload](#creation-with-upload)).
+
+Clients and servers MAY support additional well known metadata keys:
+
+* ``filename`` for a common file name
+
+The specific metadata keys documented herein are reserved for the
+respective use and MUST NOT be used for other purposes.
+
+##### [Content-Encoding](https://httpwg.org/specs/rfc7231.html#header.content-encoding)
+
+Clients MUST set the ``Content-Encoding`` header correctly IFF a
+content encoding is used.
+
+As per RFC7231, Servers MUST respond with status 415 if they can not
+accept the ``Content-Encoding`` chosen by the client.
+
+Servers MUST either store the ``Content-Encoding`` and deliver it with
+subsequent requests, or properly decode the content before storing it.
+
+##### [Content-Language](https://httpwg.org/specs/rfc7231.html#header.content-language)
+
+Clients and Servers SHOULD support the ``Content-Language`` header.
+
 #### Requests
 
 ##### POST