I was looking around and made simple patch. It is I think easiest solution that does not touch api (i.e., without adding function to filesystem that reads file and returns FileData instead of its content like filesystem.read). I tested and it seems to be working, i.e., now
t = love.thread.newThread("thread name", love.filesystem.newFileData(love.filesystem.read("filename.lua"))) }}}
t = love.thread.newThread("thread name", "filename.lua") }}}
gives same result, which seems intended (looking at string clause that was already there).