This paper targets the application of cloud storage management for mobile devices. Because of the limit of bandwidth and other resources, most existing cloud storage apps for smartphones do not keep local copies of files. This efficient design, however, limits the application capacities. In this paper, our goal is to extend the available file operations for cloud storage service to better serve smartphone users. We develop Skyfiles, an efficient and secure file management system that supports more advance file operations. Our basic idea is to utilize cloud instances to assist file operations. Particularly, Skyfiles supports download, compress, encrypt, convert operations, and file transfer between two smartphone users' cloud storage spaces. In addition, we design protocol for users to share their idle instances.