Fix: cannot write mode RGBA as JPEG (#11102)

### What problem does this PR solve?
Fix #11091 
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/app/picture.py

@@ -97,7 +97,13 @@ def vision_llm_chunk(binary, vision_model, prompt=None, callback=None):
 
     try:
         with io.BytesIO() as img_binary:
-            img.save(img_binary, format="JPEG")
+            try:
+                img.save(img_binary, format="JPEG")
+            except Exception:
+                img_binary.seek(0)
+                img_binary.truncate()
+                img.save(img_binary, format="PNG")
+                
             img_binary.seek(0)
             ans = clean_markdown_block(vision_model.describe_with_prompt(img_binary.read(), prompt))
             txt += "\n" + ans