set sourceFolder to choose folder

tell application "Finder"
set theFolders to get folders of folder sourceFolder

repeat with theFolder in theFolders
if (count of (every file of theFolder)) is 1 then
set newName to get name of theFolder
set fileName to every file of theFolder as alias
set theExt to name extension of fileName
set name of fileName to newName & "." & theExt
end if
end repeat

end tell