[MIME] Expand default extension map

[NickNevzorov]

I am using packages, that depends on mime (for example, DropTarget). I need to add my file extension to default map, but it is Const

I propose change to Final, now my code is:

import 'package:mime/mime.dart' as mime;
mime.defaultExtensionMap['wxo'] = 'application/wxo';

This will give the opportunity to add own extension types in standard resolver

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.