enhancement: have option for KB clients to recognize http(s) hyperlinks to known filetypes in chat message and convert to direct-upload

while the request for chat to include automatic gif/hyperlink unfurling includes certain privacy infringing ramifications, a potential solution to the simple task of auto-displaying images could be to have the client locally recognize hyperlinks, see if they end with known filetypes like .gif, .png, .jpg etc. and initiate a local download, then upload to the keybase chat thread.

this would eliminate the keybase servers ever having to know or see the contents of the encrypted chat, while still allowing for a more comparable chat experience to other platforms. it also could be leveraged to streamline uploading files within chat for other formats, for example a direct link to a .pdf file on a site would no longer require the person receiving that message to click the link to the document, but could download it directly from keybase while maintaining full conversation encryption.

The issue names no files or tests. Begin by locating chat message link handling and the local download/upload entry points across the client, then define completion as recognizing supported filetype links, uploading the content into the chat thread, and preserving the stated end-to-end privacy goal.