First check permission on store where you getting file,

if you have permission try put N before files

@file_attachments = N'\\waccounting\ffwin\images\vendor\ap invoice\13085316.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085317.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085320.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085327.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085338.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085339.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085341.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085344.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085347.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085352.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13086164.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085353.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085358.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085361.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085362.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085365.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085366.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085368.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085369.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085370.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085396.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085437.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085447.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085451.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085473.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085561.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13085349.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13086340.TIF;\\waccounting\ffwin\images\vendor\ap invoice\13086350.TIF'

The size of attachment is limited to 1 MB per file. Check it in Database Mail configuration