Add topics to pubspec.yaml (dart-archive/mime#121)

pkgs/mime/pubspec.yaml

@@ -4,6 +4,10 @@ description: >-
   Utilities for handling media (MIME) types, including determining a type from
   a file extension and file contents.
 repository: https://github.com/dart-lang/mime
+topics:
+ - mimetype
+ - magic-numbers
+ - multipart-form
 
 topics:
  - mime