When you say you lost a flv file, do you mean;
1) It's on your pc somewhere, but you can't find it, or
2) You had it on pc, but you deleted it, but now want it back ?
If it's 1) just type a wildcard followed by flv and search
e.g. in search bar type, *.flv (enter)
If it's 2) You need to use some file recovery software, then search for 'movie types'.