Dropbox, the San Francisco-based cloud storage and file sharing company, is expected to go public this month under auspicious circumstances: The stock market is rebounding and the economy continues to show strength. But its IPO could prove to be a “down round” — where the value falls below the amount previously set in the last private financing round. In 2014, Dropbox was privately valued at billion, but its prospectus now puts the new valuation at around billion, according to a recent filing with the Securities and Exchange Commission. What happened? “That $10 billion figure, as with other unicorns, is an overestimate of what the private market was actually willing to pay,” said Jay Ritter, a finance, insurance and real estate professor at the University of Florida. Also, “late-round investors [got] various protections that the earlier investors and employees with stock options or restricted stock didn’t have.…