I use Jotterpad on Android to edit markdown and text files. It syncs well with Dropbox (it feels just like a frontend for a dropbox folder) and supports .txt, .md, .markdown, and .fountain files.

I use Brackets and Typora to edit .md files on pc/mac/linux. I find image handling in .md files fairly simple as long as you maintain an appropriate folder structure on the backend. Typora makes this especially easy.

I would like to build a better self-hosted tagging/viewing program for my huge cache of notes - something like Google Keep without being, well, Google. Turtl comes closest to my needs, but the lack of import/export at this point is a problem. I do not wish to be tied to a particular format or storage space.

What about WriterPlus? I've been using it for years to edit my Markdown files on Android. (I was about to say that it's also open source but that seems to have changed recently.)

Also, what do you mean by "Dropbox-aware"? It's all just files after all.

Exactly- it is the best I have found so far. However, the main problem hides in everything to do with images, as they cannot be appended to a text document. Because of that, I am using this naming scheme: example.txt needs an appended image. In the doc, I write “[0]”, and create an image named example-0.jpg.