doc update

docs/Fields.md

@@ -165,9 +165,17 @@ Date and time selector. Stores as an ISO 8601 string.
 
 ---
 
+### uuid
+
+UUID text field. Stores an arbitrary UUID string. Typically used as a read-only external reference ID.
+
+**Required:** `name`, `label`
+
+---
+
 ### json
 
-Raw JSON data field.
+Raw JSON data field. Accepts either a JSON string or a plain array — both are stored as JSON.
 
 **Required:** `name`, `label`
 
@@ -175,7 +183,7 @@ Raw JSON data field.
 
 ### file
 
-Upload or select files from a files schema.
+Upload or select files from a files schema. Files can be uploaded directly or browsed from previously uploaded files. Image files (jpeg, png, webp, gif, tiff) automatically get a 300×300 thumbnail generated on upload.
 
 **Required:** `name`, `label`, `collections`