Clarify contents api further

docs/source/extending/contents.rst

@@ -81,7 +81,9 @@ model. There are three model types: **notebook**, **file**, and **directory**.
     - The ``format`` field is either ``"text"`` or ``"base64"``.
     - The ``mimetype`` field can be any mimetype string, but defaults to 
       ``text/plain`` for text-format models and
-      ``application/octet-stream`` for base64-format models.
+      ``application/octet-stream`` for base64-format models. For files with
+      unknown mime types (e.g. unknown file extensions), this field may be
+      `None`.
     - The ``content`` field is always of type ``unicode``.  For text-format
       file models, ``content`` simply contains the file's bytes after decoding
       as UTF-8.  Non-text (``base64``) files are read as bytes, base64 encoded,
@@ -100,9 +102,10 @@ model. There are three model types: **notebook**, **file**, and **directory**.
    In certain circumstances, we don't need the full content of an entity to
    complete a Contents API request.  In such cases, we omit the ``content``, and
    ``format`` keys from the model, and the default values for the ``mimetype``
-   field. This most commonly occurs when listing a directory, in which
-   circumstance we represent files within the directory as content-less models
-   to avoid having to recursively traverse and serialize the entire filesystem.
+   field (see above). This most commonly occurs when listing a directory, in
+   which circumstance we represent files within the directory as content-less
+   models to avoid having to recursively traverse and serialize the entire
+   filesystem.
 
 **Sample Models**