[BUG] No upload progress shown when attaching large files in Conversation; UI appears frozen

[epamLDadayan]

Title: No upload progress shown when attaching large files in Conversation; UI appears frozen

Description:
When attaching large files to a conversation, no upload progress indicator is shown. For several seconds (depending on file size), the Conversation page appears unresponsive, leading users to believe the process or application has frozen.

Steps to reproduce:

1. Start or open a conversation.
2. Attach a large file (e.g., 5MB+ PDF, DOCX, PowerPoint, etc.).
3. Observe the UI immediately after selecting the file.

Actual Result:

• No upload progress bar or indicator is shown.
• The Conversation page appears frozen or stuck for several seconds.
• Users have no feedback that the upload is in progress.

Expected Result:

• Display an upload progress bar (similar to the one used in Artifacts → Buckets when uploading files).
• Show percentage completion and file name during upload.
• Provide clear visual feedback that the file is being uploaded and processed.
• Once upload completes, transition smoothly to the indexing phase (if applicable) or allow the user to send the message.

Notes (optional):

• Screenshot shows the upload progress bar from Artifacts menu (15% for "AI Agents (2025).pdf"); reuse this component in Conversations.
• Improves UX by preventing confusion and reassuring users that the system is working.
• Applies to all file attachments in Chat, regardless of size, but especially critical for large files.

[epamLDadayan]

@HawkQing1 this is "connected" issue to this one #2380

[epamLDadayan]

@HawkQing1 As we discussed, This is actual for New Conversations, for now I will add it into RNs.