IsValidFile fails on aiff files generated by the Mac OSX `say` command.

Thanks for your work in this, its great.

Here is a sample file, you can run it on the aifftowav sample code in this repository and IsValidFile returns false. I had to zip it, because GitHub doesnt allow aiff files.

If you comment out the IsValidFile check, the aifftowav command works fine.

melina~ακ.aiff.zip
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[y-71]

i've filed a draft PR to try to resolve it,

it failed to read header - format not supported - [4]uint8{0x52, 0x49, 0x46, 0x46}

#5