In the "Handle or DLL substring:" text box, type the path to the file (e.g. "C:\path\to\file.txt") and click "Search". All processes which have an open handle to that file should be listed. WhoLockMe - Explorer extension which adds a right-click menu option More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.