Transferring file with Dropbox when it gets cut in the middle

Solution 1:

It depends on the action, if you're uploading a completely new file and the upload is suddenly interrupted, you're required to re-upload the entire file. If you're syncing files and the the sync is currently interrupted, you will only need to sync the parts of the file that have not been synced yet because Dropbox uses Delta Encoding technology:

Delta encoding is a way of storing or transmitting data in the form of differences between sequential data rather than complete files; more generally this is known as data differencing. Delta encoding is sometimes called delta compression, particularly where archival histories of changes are required (e.g., in software projects).

UPDATE

I've been playing with this a bit and it seems to vary, sometimes I'm forced reupload the entire thing, sometimes it just resumes...

References:
http://en.wikipedia.org/wiki/Delta_encoding
http://en.wikipedia.org/wiki/Dropbox_%28service%29#Functionality
Own experience