Bug: Uploading existing file overwrites file in place with no warning

[windrag]

1. Upload file
2. Modify file on local machine
3. Upload file with same name but new modification time
4. Existing file on server is overwritten with no warning

[nrj]

I wanted to get this into the 0.5 release but it was a bit trickier than expected. Will definitely be in the next minor release.