add note about how MIME types are detected

docs/open_actions.rst

@@ -103,7 +103,8 @@ lines. The various available criteria are:
     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``. Note that the MIME type for directories is
-    ``inode/directory``.
+    ``inode/directory``. MIME types are detected based on file extension, not
+    file contents.
 
 ``ext``
     A comma separated list of file extensions, for example: ``jpeg, tar.gz``