Added mime type change for .gif files

[doekenorg]

Description of the Change

Screenshots can be .gif files. These are useful for showing the interaction the user can expect. The mime-type of .gif files is not updated. This PR adds that.

Alternate Designs

None. I can do it by hand every time 😄

Benefits

.gif files in the assets-folder will automatically have the correct mime type, and this will prevent people from downloading the file, instead of viewing it in the browser.

Possible Drawbacks

If you really want people to download the .gif files, this would prevent that. I think the chances of this are slim to non-existent.

Verification Process

There is really nothing to verify; other than that you can actually use .gif as a screenshot. Take a look at our screenshot section for an example.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

• Mime type of .gif asset files are not updated #75

Changelog Entry

### Added
- Added mime type change to `image/gif` for `.gif` files.

[dinhtungdu]

LGTM!

[iamdharmesh]

LGTM! Thanks for the PR.