fix: refuse oversized TIFF/BMP attachments before converting them

A deflate-compressed TIFF under 1 MB can declare 144 million pixels and
take 1.2 GB to convert to PNG, and Pillow only warns below 179 million.
Read the dimensions from the header and refuse images over 40 million
pixels before any pixel data is decoded. Pillow's DecompressionBombError
is now raised as DocumentParseError, so the upload fails once instead of
being retried.

docsgpt/parser/file/image_parser.py

@@ -18,6 +18,12 @@ from docsgpt.parser.file.base_parser import BaseParser, DocumentParseError
 # to PNG before it is stored for the model.
 VISION_CONVERTIBLE_MIME_TYPES = frozenset({"image/tiff", "image/bmp", "image/x-ms-bmp"})
 
+# Largest image re-encoded to PNG. A deflate TIFF under 1 MB can declare
+# 144 million pixels and take 1.2 GB to convert, while Pillow only warns below
+# 179 million. Vision models downscale far below this cap (Anthropic refuses
+# more than 8000 px per side), so a larger image gains nothing.
+MAX_CONVERTIBLE_PIXELS = 40_000_000
+
 
 def convert_image_to_png(file_obj: BinaryIO) -> Tuple[bytes, int]:
     """Re-encode the first frame of an image as PNG.
@@ -30,12 +36,20 @@ def convert_image_to_png(file_obj: BinaryIO) -> Tuple[bytes, int]:
         source had. Only the first is kept.
 
     Raises:
-        DocumentParseError: If the bytes are not an image Pillow can decode.
+        DocumentParseError: If the bytes are not an image Pillow can decode,
+            or the image is larger than ``MAX_CONVERTIBLE_PIXELS``.
     """
     from PIL import Image, UnidentifiedImageError
 
     try:
         with Image.open(file_obj) as image:
+            # Opening reads only the header; refuse before any pixels decode.
+            width, height = image.size
+            if width * height > MAX_CONVERTIBLE_PIXELS:
+                raise DocumentParseError(
+                    f"The image is too large to convert ({width}×{height} pixels; the limit is "
+                    f"{MAX_CONVERTIBLE_PIXELS // 1_000_000} million pixels). Resize it and upload it again."
+                )
             frames = getattr(image, "n_frames", 1)
             image.seek(0)
             frame = image
@@ -43,7 +57,7 @@ def convert_image_to_png(file_obj: BinaryIO) -> Tuple[bytes, int]:
                 frame = frame.convert("RGBA" if "A" in frame.getbands() else "RGB")
             out = io.BytesIO()
             frame.save(out, format="PNG")
-    except (UnidentifiedImageError, OSError) as exc:
+    except (UnidentifiedImageError, OSError, Image.DecompressionBombError) as exc:
         raise DocumentParseError(f"Could not read the image: {exc}") from exc
     return out.getvalue(), frames