NIMMessageAttachmentStatus constructor Null safety

1. nim_core
2. nim_core
3. NIMMessageAttachmentStatus
4. NIMMessageAttachmentStatus const constructor

NIMMessageAttachmentStatus enum

1. Constructors
2. NIMMessageAttachmentStatus
3. Values
4. initial
5. failed
6. transferring
7. transferred
8. cancel
9. Properties
10. hashCode
11. index
12. runtimeType
13. Methods
14. noSuchMethod
15. toString
16. Operators
17. operator ==
18. Constants
19. values