Convert images to PNG before sending to llama-server

llama-server uses stb_image internally which does not support WebP,
TIFF, AVIF, and other formats that browsers accept for upload.
Uploading a WebP image to a vision GGUF model caused a 400 error:
"Failed to load image or audio file" / "failed to decode image bytes".

Convert all uploaded images to PNG via PIL before base64-encoding and
forwarding to llama-server. This handles WebP, TIFF, BMP, GIF, AVIF,
and any other format PIL supports. RGBA images are converted to RGB
first since PNG with alpha can cause issues in some vision pipelines.

studio/backend/routes/inference.py

@@ -860,6 +860,23 @@ async def openai_chat_completions(
                 detail = "Image provided but current GGUF model does not support vision.",
             )
 
+        # Convert image to PNG for llama-server (stb_image has limited format support)
+        if image_b64:
+            try:
+                import base64 as _b64
+                from io import BytesIO as _BytesIO
+                from PIL import Image as _Image
+
+                raw = _b64.b64decode(image_b64)
+                img = _Image.open(_BytesIO(raw))
+                if img.mode == "RGBA":
+                    img = img.convert("RGB")
+                buf = _BytesIO()
+                img.save(buf, format = "PNG")
+                image_b64 = _b64.b64encode(buf.getvalue()).decode("ascii")
+            except Exception as e:
+                raise HTTPException(status_code = 400, detail = f"Failed to process image: {e}")
+
         # Build message list with system prompt prepended
         gguf_messages = []
         if system_prompt: