Feature Request: Support attaching local files (PDF, Word, Excel, Markdown, Text) as Chat Context (RAG)

Summary
Please add the ability to upload or reference local files (such as PDFs, Word documents, Excel spreadsheets, Markdown, and various text formats) as context (Retrieval-Augmented Generation / RAG) in the Eclipse GitHub Copilot Chat panel. This would allow developers to feed requirements, data sheets, and documentation directly to Copilot to guide application development.

Detailed Context & Problem
First of all, thank you for the fantastic improvements in version 0.20.0! Every update has been incredibly helpful, and the plugin keeps getting better.

Currently, when building an application, critical foundational documentation is stored in various formats—such as product requirements in Word (.docx), data structures or mappings in Excel (.xlsx), formatted documentation in Markdown (.md/.txt), or system architectures in PDFs.

While web-based LLMs (like ChatGPT, Gemini, and Claude) and competing IDE tools (such as IntelliJ's AI Assistant) allow users to upload or reference these files as immediate chat context, the Eclipse GitHub Copilot plugin is currently limited in handling non-code file formats.

Without this feature, developers must manually copy-paste text out of documents (which often breaks tables, formatting, and structures) or convert complex documents into raw text files just to get Copilot to read them.

Proposed Solutions / Feature Scope

1. Direct File Attachment in Chat (Ideal)
   Add a "paperclip" or file attachment button to the Eclipse Copilot Chat panel.

Allow users to upload or drag-and-drop local files—specifically PDF, Word, Excel, Markdown, and standard text files—so Copilot can parse their contents and use them as RAG context for the conversation.

2. Workspace File Context (Alternative)
   If a direct upload UI is not feasible, allow Copilot to index and read these document formats when they are placed directly inside the active Eclipse Project Explorer.

Users should be able to reference the document in chat (e.g., using a variable like #file:specs.xlsx or #file:requirements.docx) and ask Copilot to generate code based on the data or instructions inside those files.

Additional Question / Clarification
Does GitHub Copilot CLI or any existing Copilot tool currently support parsing or using these non-code document formats (like PDFs, Word, or Excel) as prompt context? If there is an existing CLI workaround, please let us know. Otherwise, we would love to see this integrated directly into the Eclipse IDE plugin ecosystem.

Thank you for your hard work and continuous improvements!

Start by reviewing the Eclipse GitHub Copilot Chat panel and how it currently handles non-code file formats. Compare the proposed direct attachment and workspace-file approaches, including PDF, Word, Excel, Markdown, and text files; done means supported local documents can provide usable context in chat.