Add OAS guidance for bytes fields (AEP-140) (#376)

Author: rambleraptor

aep/general/0140/aep.md.j2

@@ -138,7 +138,21 @@ base64-encode a field into a `string` field.
 
 {% tab oas %}
 
-**Note:** OAS guidance not yet written.
+When sending binary data over the wire, the field name **should** be suffixed
+with "\_bytes". The contents of the field **should** be base64-encoded over the
+wire. The field **should** be annotated with `contentEncoding: base64` and
+**may** include `contentMediaType` to specify the media type of the binary
+data.
+
+For example:
+
+```yaml
+profile_image_bytes:
+  type: string
+  contentEncoding: base64
+  contentMediaType: image/png
+  description: The user's profile image as a base64-encoded PNG.
+```
 
 {% endtabs %}