By Gerry Shih SAN FRANCISCO (Reuters) – Dropbox Inc on Wednesday unveiled what it described as one of the most comprehensive upgrades to its service for businesses, including a feature that allows users to easily maintain both personal and corporate accounts. The new features come at a time when large rivals like Microsoft Corp and Amazon Inc and smaller competitors are battling to win the cloud-storage market, which is widely seen as a strategic linchpin in the era of mobile computing. The upgrades reflect the changing business focus at Dropbox – one of the most closely-watched privately held Internet companies – toward becoming a file-sharing solution for corporate customer, a critically important and lucrative market. Dropbox, valued at $4 billion by venture capital investors, is viewed as a hot prospect for an initial public offering within the next two or three years.