Allow for PDFs to be attached

[Southporter]

Currently Battleground only allows for images to be added to a chat or a prompt. It would be great to support more file types.

The Bedrock UI allows the following:
csv, html, doc(x), pdf, md, txt, xls(x)

I think that all this requires is for useFilePicker to allow these types

 const { openFilePicker } = useFilePicker({
   multiple: false,
   accept: ["image/*", ".pdf", ".txt"], // And more could be added.
   readAs: "DataURL",
   onFilesSuccessfullySelected: ({ filesContent }) => {
     if (!filesContent || filesContent.length === 0) return;
     const imageData = { name: filesContent[0].name, dataUrl: filesContent[0].content };
     addAttachmentToPrompt(imageData);
   },
 });