ARM assembly syntax file using .s file extension conflicts with asm.yaml

Description of the problem or steps to reproduce

I would like to use micro for a project in ARM assembly language, and could contribute a syntax file for that instruction set. However, our source files use the extension .s, which is already used by the built-in syntax file for x86 assembly language. I looked at the detect/header syntax in YAML, but the Go code in ftdetect.go prefers the file extension to the firstLine header. I can see two cumbersome ways to make this work:

1. Make up a bogus file extension like .as for ARM assembly language
2. Type set filetype armasm every time I open a file this doesn't work

Is there a better way? If not, can we make something up, like maybe allowing the firstLine header to take priority over the file extension?

Specifications

Version: 2.0.2
Commit hash: c51f849
Compiled on February 26, 2020

Start by reading runtime/syntax/asm.yaml and pkg/highlight/ftdetect.go, focusing on how file extensions and firstLine headers are prioritized. Determine how ARM assembly files using .s can be distinguished from x86 assembly without requiring a bogus extension or manual filetype setting, then verify the detection behavior with relevant syntax-detection tests.