docs: Add example on how to automatically store mimetype in files table

Resolves PostgREST/postgrest-docs#612

docs/how-tos/providing-images-for-img.rst

@@ -81,9 +81,11 @@ First, in addition to the minimal example, we need to store the media types and
 .. code-block:: postgres
 
    alter table files
-     add column type text,
+     add column type text generated always as (byteamagic_mime(substr(blob, 0, 4100))) stored,
      add column name text;
 
+This uses the :code:`byteamagic_mime()` function from the `pg_byteamagic extension <https://github.com/nmandery/pg_byteamagic>`_ to automatically generate the mimetype in the :code:`files` table. To guess the type of a file, it's generally enough to look at the beginning of the file, which is more efficient.
+
 Next, we set modify the function to set the content type and filename.
 We use this opportunity to configure some basic, client-side caching.
 For production, you probably want to configure additional caches, e.g. on the :ref:`reverse proxy <nginx>`.