clarify file upload mechanism details

Author: ewdurbin

peps/pep-0694.rst

@@ -159,6 +159,7 @@ these steps:
 #. Complete the upload session, publishing or discarding the stage.
 #. Optionally check the status of an upload session.
 
+.. _versioning:
 
 Versioning
 ----------
@@ -759,26 +760,42 @@ interpretation to aid in diagnosing underlying issue.
 File Upload Mechanisms
 ----------------------
 
-File Upload Mechanisms, with the exception of ``http-post-application-octet-stream`` are left as an
-implementation detail specific to each server. Servers **MUST** implement a
-``http-post-application-octet-stream`` mechanism. This serves as a fallback if no server specific implementations
-exist.
+Servers **MUST** implement :ref:`required file upload mechansisms <required-file-upload-mechanisms>`.
+Such mechanisms serve as a fallback if no server specific implementations exist.
 
-A given server may implement an arbitrary number of mechanisms and is responsible for documenting
-their usage. Implemenatations **SHOULD** be prefixed with a string that clearly identifies the
-server and is unique from other well known servers or implementations.
+Each major version of the Upload API **MUST** specify at least one required File Upload Mechanism.
 
-If a server intendes to match the behavior of another server's implementation, it **MAY** respond
+New required mechanisms **MUST NOT** be added and existing required mechanisms **MUST NOT** be removed
+without an update to the :ref:`major version <versioning>`.
+
+A given server **MAY** implement an arbitrary number of server specific mechanisms
+and is responsible for documenting their usage.
+Server specific implementations **MUST** be prefixed with ``vnd-``
+and **SHOULD** further contain a string that clearly identifies the server operator
+and is unique from other well known servers or implementations.
+
+For example:
+
+====================================== ================  ========================================================
+File Upload Mechanism string           Server Operator   Mechanism description
+====================================== ================  ========================================================
+``vnd-pypi-s3multipart-presigned``     PyPI              S3 multipart upload via pre-signed URL
+``vnd-pypi-fetch``                     PyPI              File delivered by instructing server to fetch from a URL
+``vnd-acmecorp-fetch``                 Acme Corp         File delivered by instructing server to fetch from a URL
+``vnd-acmecorp-postal``                Acme Corp         File delivered via postal mail
+``vnd-madscience-quantumentanglement`` Mad Science Labs  Upload via quantum entanglement
+====================================== ================  ========================================================
+
+If a server intends to match the behavior of another server's implementation, it **MAY** respond
 with that implementation's file upload mechanism name.
 
-All implementations of this PEP **MUST** implement the ``http-post-application-octet-stream`` file
-upload mechanism.
+.. _required-file-upload-mechanisms:
 
-``http-post-application-octet-stream`` Mechanism
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Required File Upload Mechanisms
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The ``http-post-application-octet-stream`` mechansism **MUST** be supported servers which
-implement this PEP.
+``http-post-application-octet-stream``
+++++++++++++++++++++++++++++++++++++++
 
 A client executes this mechanism by submitting a ``POST`` request to the ``file_url`` returned in the
 ``http-post-application-octet-stream`` map of the ``mechanism`` map of the