feat(mime): add gsheet default extension to map

[ziqq]

• Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

Related issue #1198

[mosuem]

@ziqq - AFAIU, a .gsheet file cannot be opened directly. Would you mind explaining the use case of this?

[ziqq]

@ziqq - AFAIU, a .gsheet file cannot be opened directly. Would you mind explaining the use case of this?

Hello @mosuem, Yes i can. U can use mime.lookupMimeType to check files 'my_file.gsheet' of google sheets to get there mime type.