[Fix] Detect MIME types for extensionless images

[reacher-z]

Summary

• detect the MIME type of local files with unknown or missing extensions using Pillow
• keep the existing extension-based fast path unchanged
• return unknown for non-image files instead of raising
• add regression coverage for a real extensionless PNG and an extensionless non-image

This fixes the remaining SiteBenchImage failure reported in #1485. The 3DSRBench name_str problem from the same issue was already fixed by #1428.

Why

Some SiteBenchImage assets have no filename extension. parse_file currently maps an empty suffix to unknown, so API and model adapters cannot treat those paths as images even though Pillow can identify their encoded format.

Tests

• python3 -m pytest -q tests/test_smp_file.py (2 passed)
• uvx pre-commit run --files vlmeval/smp/file.py tests/test_smp_file.py (all passed)
• git diff --check

AI assistance disclosure

I used OpenAI Codex to inspect the issue and repository, implement the change, and draft tests and this description. I reviewed the code and personally ran the tests and checks above.

Closes #1485