Hi there,
Panda read and write in pure Markdown if there are no images/files attached to your document, as text files can only contain “text” 
If you add an image or a file to the document, the format is changed to .panda, which is an open format: Textpack which is a zipped TextBundle.
This is a common format used to bundle text and images/files together, there are other editors who support it, but you can always unzip it and extract your Markdown file and your attachments.
We’re trying to use open formats and let you in full control of your data, we’re also open to any suggestion on this matter.
Let me know what you think about this!
Best.