fix the way matrix audio mimetype handling works #510

Job	Run time
golangci-lint	1m 0s
test (1.24.x)	1m 46s
test (stable)	1m 59s
build-upload (amd64, linux, linux)	1m 23s
build-upload (amd64, mac, darwin)	1m 23s
build-upload (amd64, windows, windows)	1m 32s
	9m 3s

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