Improving MIME type detection for some common file types when they are missing from the system MIME database

Also allow the user to specify their own database via mime.types in the
kitty config directory. See #3056

docs/changelog.rst

@@ -24,6 +24,9 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - Fix selections created by dragging upwards not being auto-cleared when
   screen contents change (:pull:`3028`)
 
+- Allow adding MIME definitions to kitty by placing a ``mime.types`` file in
+  the kitty config directory (:iss:`3056`)
+
 
 0.19.1 [2020-10-06]
 -------------------

docs/open_actions.rst

@@ -81,7 +81,11 @@ lines. The various available criteria are:
 
 ``mime``
     A comma separated list of MIME types, for example: ``text/*, image/*,
-    application/pdf``
+    application/pdf``. You can add MIME types to kitty by creating the
+    :file:`mime.types` in the kitty configuration directory. Useful if your
+    system MIME database does not have definitions you need. This file is
+    in the standard format of one definition per line, like: ``text/plain rst
+    md``.
 
 ``ext``
     A comma separated list of file extensions, for example: ``jpeg, tar.gz``