FEATURE: Multimodal and document upload to context

[twalderman]

As a user of the Obsidian chat plugin,
I want to be able to upload various file types, including images, audio, and documents, within the chat interface
So that I can easily share diverse content with my conversation partner.

Acceptance Criteria:

• A paperclip icon or similar button is added near the chat input field to initiate file uploads.
• Clicking the upload button opens the user's file explorer to select a file.
• The plugin supports common file formats, such as:
  Images: PNG, JPEG, GIF, BMP, SVG
  Audio: MP3, WAV, OGG, M4A
  Documents: PDF, DOCX, TXT, MD
• The selected file is uploaded and displayed or linked within the chat conversation.