Add ".wasm" to AllowFileExtensions and associate MimeType "application/wasm"
WebAssembly is a new binary format for static files. The official site has more info. https://webassembly.org/
Google for webassembly mimetype w3c will return many results showing the addition of this file extension and Mimetype in other mainstream servers.
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wasm has been added to ServiceStack from v5.1.1 which is now available on MyGet:
http://docs.servicestack.net/myget