Drag and drop file + link to it

[cloverich]

This is one of several features required before I can import my Notion documents. I have documents that reference PDFs and perhaps other files, that I don't intend to render inline (i.e. no image or video preview). Instead, probably:

• Copy the file to files location
• Render a link to it

What does the link do? Doesn't render in app. Downloading doesn't make sense. Open finder to it could be nice; but then #62 gets more complicated.

UPDATE: Based on my work implementing video (#54 ) I learned a few things that will be relevant; don't want to forget.

• Generic drag and drop file handler: Both image and video plugins manage "uploading" files; the (plate) image plugin actually eats the event if its not video, so the video plugin must come after it. It may make sense to consolidate all file handling into a single plugin, that can manage implementing or delegating for images, videos, and other elements
• Need to determine how file links will be represented in markdown: As links, as image tags, or both. Then need to update the handlers (for those) in the internal remark-slate-transformer code to handle files as well.