OpenAIFile.fromJson constructor
Implementation
factory OpenAIFile.fromJson(Map<String, dynamic> j) => OpenAIFile(
id: j['id'] as String,
object: j['object'] as String, // "file"
bytes: (j['bytes'] as num).toInt(),
createdAt: (j['created_at'] as num).toInt(),
filename: j['filename'] as String,
purpose: FilePurpose.fromJson(j['purpose']), // keep as raw string for forward-compat
);