As discussed in T153315, converting pasted HTML to wikitext is not always the desired result. This can be especially annoying if the formatted HTML contains wikitext, as it will be escaped with <nowiki> tags.
We should instead prompt the user when pasting formatted HTML, and force them to deliberately choose the "convert to wikitext" option, with the default being plain-text (and possibly a reminder about the CTRL+SHIFT+V shortcut?).
This behaviour is similar to the paste dialog shown in excel/calc when the source format can't be inferred (e.g. "CSV or TSV?")