[EOSIO/eos] Error 3160009: No wast file found (#5349)

I use latest eosio/eos-dev docker images ,and jungletestnet http://jungle.cryptolions.io:18888
with helloworld contract,meet following errors:

cleos set contract helocode1234 ./helo -p helocode1234@active
Reading WASM from ./helo/helo.wasm…
Error 3160009: No wast file found
Error Details:
no wasm file found ./helo/helo.wasm

but these files do exist:
ls ./helo/
helo.abi helo.cpp helo.wasm helo.wast

why?