enhance: Show codec names in table preview instead of =BLOB=

- Raw blobs (no codec) now show "bytes"
- Raw json (no codec) shows "json"
- Codec fields show "<codec_name>" (e.g., <blob>, <hash>, <object>)
- Improves clarity when viewing table contents

Example output:
  *id    raw_blob  blob_data  json_data
  1      bytes     <blob>     json

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/preview.py

@@ -24,6 +24,18 @@ def _format_object_display(json_data):
         return "=OBJ[file]="
 
 
+def _get_blob_placeholder(heading, field_name):
+    """Get display placeholder for a blob/json field based on its codec."""
+    attr = heading.attributes.get(field_name)
+    if attr is None:
+        return "bytes"
+    if attr.codec is not None:
+        return f"<{attr.codec.name}>"
+    if attr.json:
+        return "json"
+    return "bytes"
+
+
 def preview(query_expression, limit, width):
     heading = query_expression.heading
     rel = query_expression.proj(*heading.non_blobs)
@@ -55,7 +67,7 @@ def preview(query_expression, limit, width):
     def get_placeholder(f):
         if f in object_fields:
             return "=OBJ[.xxx]="
-        return "=BLOB="
+        return _get_blob_placeholder(heading, f)
 
     widths = {
         f: min(
@@ -72,7 +84,7 @@ def get_display_value(tup, f, idx):
         elif f in object_fields and idx < len(object_data_list):
             return _format_object_display(object_data_list[idx].get(f))
         else:
-            return "=BLOB="
+            return _get_blob_placeholder(heading, f)
 
     return (
         " ".join([templates[f] % ("*" + f if f in rel.primary_key else f) for f in columns])
@@ -113,7 +125,7 @@ def get_html_display_value(tup, name, idx):
         elif name in object_fields and idx < len(object_data_list):
             return _format_object_display(object_data_list[idx].get(name))
         else:
-            return "=BLOB="
+            return _get_blob_placeholder(heading, name)
 
     css = """
     <style type="text/css">