Allow storing files #44
Description
Allow storing files in Edna (not available when using local storage).
At first limit to known file types (image files, pdf, zip, .txt, .md, programming languages etc.).
At first we would just show them in file-specific UI e.g. show images / pdf in a dialog, .zip files would list content etc.
Later on we would allow displaying images inline either as a block or in markdown blocks when linked.
Maybe store files in files sub-directory.
To store a file, user can drag&drop or just copy it to files sub-directory.
When drag&drop we ensure the name is unique and record original name in metadata.
For opening images, a custom picker that shows thumbnails.