Resolve "Load only metadata for Markdown video include"

What does this MR do?

Adds the preload attribute to the uploaded videos, to fetch only the video metadata to prevent any unnecessary buffering

Closes #321297 (closed)

Screenshots (strongly suggested)

Before	After

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

Related to #321297 (closed)