However since that content can be quite large, you also have the option to store it outside the database
How big data should be to prefer filesystem over DB? Is this depends from attachments size or all data? What downsides of using filesystem from the start? Performance drop or something other too?
In general databases are really fast with small entries and get slower if the columns get too big, filesystems are a lot slower as indexing is not provided, but they are able to better handle big files. I think for normal users the change to the filesystem option isn't worth as a few large attachments don't harm the speed of the database much, but the speed of the filesystem should be noticeable slower. If you don't have many but huge (there isn't a hard edge considering file size) notes or attachments, you could consider it, but then maybe Joplin isn't even quite the right application for you, as of my knowledge the Joplin Clients are also based on databases.