Tinderbox does support drag/drop of direct import of styled text (.rtf) text files into the view pane.
Styled text can be pasted into note $Text, supported style and weblinks are retained. Use Paste-and-Match-Style (⌘+⌥+⇧+V) to paste a plain text version of the source text.
Styled text can be copied from $Text.
There is no bulk import method, i.e. for one or more discrete files. Note that bulk text import is supported for plain text.
See also—notes linking to here: