Windows only: Free application FragExt adds an entry to your right-click context menu so you can defragment individual files on demand. To use it, just right-click the file you want to defrag, then select FragExt -> Defragment. You can also run reports on the fragmentation of individual files and copy it to your clipboard, a file, or just view it in a newly minted tab in the file properties. If defragmentation is a major issue for you, your best bet is to find a good defrag app and set up automated runs. If you just need to defrag a large file every now and then, FragExt or the previously mentioned Defraggler will do the job nicely.