Step 1 - Update IDE to track mimeType

• POC
• Step 1 - Update IDE to track mimeType
• Step 2 - Use arraybuffer for binary files in Web IDE
• Step 3 - Use ide.files.change for live preview
• Step 4 - Use gitlab-smooshpack behind feature flag

What does this MR do?

This MR updates the Web IDE to track the mimeType we get from the BE when requesting file info, and also when we upload files. This is needed for the Live Preview feature, so that we can generate correct responses with accurate mime types.

Screenshots (strongly suggested)

Uploads still work

Opening binary and regular files still work

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Not needed because no user-facing change.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content