Dropbox Inc., the file-sharing company that’s been privately valued at $10 billion, filed for a U.S. initial public offering, saying it was at scale and capable of moving fast. The company filed with an offering size of $500 million, according to a filing with the U.S. Securities and Exchange Commission. That amount is a placeholder and is likely to change. The company plans to use the proceeds to pay down debt and for general corporate purposes.