Render WebP attachments as image by default

What does this MR do?

This MR adds the .webp file extension to the list of safe image extensions in the file type detection. It allows webp-images to be uploaded to issues and comments (via "Attach a file" or drag and drop) which then get embeded as images instead of attachments (see #296600 (closed)). It also allows uploading of designs in the webp format. WebP is supported in all browsers that GitLab supports except Safari <= macOS 10 (see caniuse, suppported browsers).

Related to #296600 (closed)

Screenshots (strongly suggested)

Feature	Before	After
Issue Description
Designs

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team