Edit file in place - alternatives to S3

So, i'm searching for cloud storage, which will allow me to update files in place (if it is possible at all). Typical task is - client opens file with his mobile device, makes some changes and saves. I was looking for S3 at first, but i allows only deleting and creating of files. For any other possibilities, as i understand, there must be some layer between S3 and client. So, maybe someone can advice some opensource project, which provides additional operations over S3. Or, even, some solutions, which can replace S3 at all (like Azure storage or cloud storage from Google)