update mediaType observations

content/tech/activitypub/!media-type.md

@@ -1,6 +1,10 @@
 +++
 +++
 
+{{<toc>}}
+
+## interpretation of request/response types in as2/ap
+
 AS2-Core says
 
 > An Activity Streams Document is a JSON document whose root value is an Activity Streams Object of any type [...] and whose MIME media type is `application/activity+json` [...] Implementations SHOULD consider the `application/ld+json; profile="https://www.w3.org/ns/activitystreams"` media type as being equivalent to `application/activity+json`.
@@ -22,6 +26,8 @@ and on the response side:
 
 - return activity+json (regardless of request type)
 
+## both are equivalent (and used in practice!)
+
 HOWEVER: in practice, you will probably need to be able to handle both types in both request+response
 
 impls that return activity+json:
@@ -33,3 +39,21 @@ impls that return ld+json:
 
 - misskey
 - pixelfed
+
+impls that POST with activity+json:
+
+- mastodon
+- ...
+
+## gotcha: multiple types
+
+ALSO NOTE: you may encounter multiple accept/content-type values. mastodon for example makes GET requests with `Accept: application/activity+json, application/ld+json` (in violation of the activitypub spec) or `Accept: application/activity+json, application/ld+json; profile="https://www.w3.org/ns/activitystreams", text/html;q=0.1` (also in violation of activitypub). these are still valid by RFC 9110 "HTTP Semantics" `Accept:` header parsing rules. note that multiple types are separated by a comma (`,`) but you must not parse them in order; *you parse them by most specific type first*.
+
+> for example:
+> `Accept: text/*, text/plain, text/plain;format=flowed, */*`
+> [has] the following precedence:
+> 1. text/plain;format=flowed
+> 2. text/plain
+> 3. text/*
+> 4. \*/\*
+