There are a few inconsistencies across browsers.

The following code that starts on line 86 of the markdown_editor_drop_zone.tsx makes the drag and drop file not accepted don't work on Safari.

accept: dropHandlers.reduce<string[]>((accept, dropHandler) => {
  accept.push(...dropHandler.supportedFiles);
  return accept;
}, []),

• I changed the background to blue when we're dragging a file. In Safari without the above code, the blue (.euiMarkdownEditorDropZone--isDragging) would always appear. And then if a file was not accepted the footer error would appear. So having blue and that error in red makes more sense than green and red. So we just assume the blue as an isDragging action rather than success.
• The .euiMarkdownEditorDropZone--isDraggingError is a red background and it disappears once we stop dragging an unaccepted file.
• The .euiMarkdownEditorDropZone--hasError' is the footer error and it stays there until we focus the textarea or try to upload/drag a new file.