feat(NewMessageUploadEditor) - caption to file share

[Antreesy]

☑️ Resolves

• Fix Frontend "File caption" #10414

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After

🚧 Tasks

• To check
  • User mentions don't render for me in NewMessage (known bug)
  • Caught it once, the order of shared files was different, because they are now being loaded in parallel
• Follow-up
  • Support multiple file shares (requires a lot of rework in frontend/backend) - draft at feat(NewMessageUploadEditor): multiple files sharing support 🖼️+🖼️ #8949

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[DorraJaouad]

Tested 🦅.

Good to go.