Markdown files with attachments cannot be edited locally

At the moment media files embedded in a markdown file are stored inside a hidden folder named .attachments.123456 next to the markdown file. There are many nice markdown editors for Mac which could be used to edit local files, but unfortunately they all are not able to scan hidden files, so the attachments appear broken.

There could be a per-file setting, e.g. "non-hidden attachments folder", also warning that the folder should not be touched. When I removed the hiding dot from the attachments folder and in the markdown source code on my local machine, i wasn't able to share the file anymore with guests, as images xouldn't be found in the shared file view. (I guess we automatically add the same permissions of the markdown-file to each attachment, but I couldn't find any documentation about the exact voodoo going on here.)

No files, tests, or entry points are named. Start by tracing how .attachments folders are created, discovered, and permissioned for shared views; determine whether a per-file non-hidden-folder setting is feasible. Done means local editors can find attachments without breaking guest sharing.