You can already drag files from the Explorer, editor tabs, or Finder into the Agent input to attach them. Dragging attached file references to reorder them is not currently supported, though you can cut and paste a reference to reposition it.
On macOS, select a file in the Explorer and press Cmd+Option+C to copy its path, or Cmd+Shift+Option+C for its relative path. You can then paste it into Agent. There is not currently a single shortcut that inserts the path directly into the prompt.
For Markdown files, Preview is available after opening the file, but the Changes/diff view does not currently offer a direct jump to Preview.