Well, ".asm" didn't help. Tried MS included "MASM" macro assembler, but I don't think it was very happy with a file only containing a macro, and it bace even more upset when I tried to feed it aes.asm.
NT zoka.bortas.org $ sprsh ml aes.s Microsoft (R) Macro Assembler Version 7.10.3077 Copyright (C) Microsoft Corporation. All rights reserved.
Assembling: aes.s aes.s(52) : error A2008: syntax error : . aes.s(133) : error A2088: END directive required at end of file