Document MIME types in specs/cofiguration

Author: krassowski

CONTRIBUTING.md

@@ -343,7 +343,8 @@ A spec is a Python callable (a function, or a class with `__call__` method) that
   "python-language-server": {            # the name of the implementation
       "version":  SPEC_VERSION,          # the version of the spec schema (an integer)
       "argv": ["python", "-m", "pyls"],  # a list of command line arguments
-      "languages": ["python"]            # a list of languages it supports
+      "languages": ["python"],           # a list of languages it supports
+      "mime_types": ["text/python", "text/x-ipython"]
   }
 }
 ```

docs/Configuring.ipynb

@@ -66,8 +66,15 @@
     "    `mkdir \"new directory\"` should be split into `[\"mkdir\", \"new directory\"]`;\n",
     "    If you have Python installed, you can use `shlex.split(\"your command\")` to\n",
     "    get such an array.\n",
-    "- the `languages` which the server will respond to, and\n",
+    "- the `languages` which the language server will respond to, and\n",
     "- the schema `version` of the spec (currently `2`)\n",
+    "- `mime_types` by which the notebooks and files will be matched to the langauge\n",
+    "  server:\n",
+    "  - for notebooks the MIME type is derived from `language_info`/`mimetype`\n",
+    "    element of [`kernel_info`][] response (with fallback on to cell metadata if\n",
+    "    missing from kernel response)\n",
+    "  - for files the implemntation is frontend-specific; in JupyterLab the MIME\n",
+    "    type is obtained from the MIME type registry.\n",
     "\n",
     "```python\n",
     "# ./jupyter_server_config.json                   ---------- unique! -----------\n",
@@ -80,12 +87,16 @@
     "      \"a-language-server-implementation\": {\n",
     "        \"version\": 2,\n",
     "        \"argv\": [\"/absolute/path/to/a-language-server\", \"--stdio\"],\n",
-    "        \"languages\": [\"a-language\"]\n",
+    "        \"languages\": [\"a-language\"],\n",
+    "        \"mime_types\": [\"text/language\", \"text/x-language\"]\n",
     "      }\n",
     "    }\n",
     "  }\n",
     "}\n",
-    "```"
+    "```\n",
+    "\n",
+    "[`kernel_info`]:\n",
+    "  https://jupyter-client.readthedocs.io/en/stable/messaging.html#kernel-info"
    ]
   },
   {