Fall back to content-based MIME detection via puremagic

[lavr]

Summary

• When file extension is missing or unrecognized, detect MIME type from file content using magic bytes via puremagic
• Extension-based detection still takes priority
• Falls back to application/unknown if neither method works
• Added puremagic to base dependencies

Closes #163

Test plan

• All 87 tests pass locally
• New tests: PNG bytes without extension, JPEG bytes without extension, unknown bytes fallback, extension priority over content