Feat: add image preview to retrieval test. (#7610)

### What problem does this PR solve?

#7608

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

rag/app/manual.py

@@ -265,7 +265,9 @@ def chunk(filename, binary=None, from_page=0, to_page=100000,
         res = tokenize_table(tbls, doc, eng)
         for text, image in ti_list:
             d = copy.deepcopy(doc)
-            d['image'] = image
+            if image:
+                d['image'] = image
+                d["doc_type_kwd"] = "image"
             tokenize(d, text, eng)
             res.append(d)
         return res