Description

base64.b64encode() returns a bytes object (e.g., b"Zm9v"). The function uses typing.cast(str, ...) which only satisfies the static type checker but does not actually convert the bytes to a string at runtime. Because the function signature advertises a str return type, downstream code will likely attempt to concatenate it with other strings (e.g., f"data:image/jpeg;base64,{img}") or serialize it to JSON for API requests. This will result in runtime errors like TypeError: Object of type bytes is not JSON serializable.

Severity: high
File: llama-index-core/llama_index/core/img_utils.py

Expected Behavior

The code should handle this case properly to avoid unexpected errors or degraded quality.