"""Shared media type detection utilities."""
import mimetypes
_IMAGE_MIME_TYPES = frozenset(
{
"image/png",
"image/jpeg",
"image/gif",
"image/webp",
"image/svg+xml",
"image/bmp",
}
)
_AMBIGUOUS_MIME_TYPES = frozenset({"application/octet-stream", "application/binary"})
_IMAGE_EXTENSIONS = frozenset(
{".png", ".jpg", ".jpeg", ".gif", ".webp", ".svg", ".bmp"}
)
def is_image_attachment(
media_type: str | None, filename: str | None
) -> tuple[bool, str]:
"""Detect whether an attachment is an image.
Uses two-tier detection: explicit MIME type check, then filename
extension fallback for ambiguous or missing MIME types.
Args:
media_type: The MIME type reported by the API.
filename: The attachment filename.
Returns:
Tuple of (is_image, resolved_mime_type).
"""
if media_type and media_type in _IMAGE_MIME_TYPES:
return True, media_type
if (media_type in _AMBIGUOUS_MIME_TYPES or media_type is None) and filename:
ext = "." + filename.rsplit(".", 1)[-1].lower() if "." in filename else ""
if ext in _IMAGE_EXTENSIONS:
guessed = mimetypes.guess_type(filename)[0] or "image/png"
return True, guessed
return False, media_type or "application/octet-stream"
