fix(attachments): Placeholder content-type (#17051)

Add a +json suffix to the attachment placeholder content type to signal
that it is syntactically JSON. The old value is still parsed correctly
for compatibility.

Author: jjbayer

develop-docs/sdk/telemetry/attachments.mdx

@@ -178,7 +178,7 @@ Trace attachments use item type `"attachment"` with content type `"application/v
 Attachment placeholders are an experimental feature, the protocol is subject to change.
 </Alert>
 
-Attachment placeholders use item type `"attachment"` with content type `"application/vnd.sentry.attachment-ref"`. A placeholder contains a **reference** to an attachment uploaded elsewhere — it does _not_ contain the actual attachment payload. The purpose is for the attachment to be handled (rate limited, filtered, etc.) together with the event payload, even if it was uploaded separately.
+Attachment placeholders use item type `"attachment"` with content type `"application/vnd.sentry.attachment-ref+json"`. A placeholder contains a **reference** to an attachment uploaded elsewhere — it does _not_ contain the actual attachment payload. The purpose is for the attachment to be handled (rate limited, filtered, etc.) together with the event payload, even if it was uploaded separately.
 
 SDKs **SHOULD** send placeholders in the same envelope as the event the file is attached to.
 
@@ -250,7 +250,7 @@ SDKs **SHOULD** send placeholders in the same envelope as the event the file is
 | Field | Type | Required | Description |
 |-------|------|----------|-------------|
 | `type` | String | **REQUIRED** | **MUST** be `"attachment"`. |
-| `content_type` | String | **REQUIRED** | **MUST** be `"application/vnd.sentry.attachment-ref"`. |
+| `content_type` | String | **REQUIRED** | **MUST** be `"application/vnd.sentry.attachment-ref+json"`. |
 | `length` | Integer | **REQUIRED** | Size of the placeholder payload in bytes. |
 | `attachment_length` | Integer | **REQUIRED** | Size of the referenced attachment in bytes. |
 | `filename` | String | **REQUIRED** | The name of the uploaded file without a path component. |
@@ -387,7 +387,7 @@ Hello World!
 
 ```
 {"event_id":"9ec79c33ec9942ab8353589fcb2e04dc"}
-{"type":"attachment","content_type":"application/vnd.sentry.attachment-ref","length":123,"attachment_length":212341234,"filename":"myfile.log"}
+{"type":"attachment","content_type":"application/vnd.sentry.attachment-ref+json","length":123,"attachment_length":212341234,"filename":"myfile.log"}
 {"location":"/api/42/upload/019c7a950dd376a1817a9ced5cb7c4b5/?length=212341234&signature=Zct1IMmM3BIJrzDOwG3tUn5AlrLhqIJFBu8Kd59dXCz","content_type":"text/plain"}
 ```