@UnstableApi
    public FileServiceBuilder fallbackFileExtensions(Iterable<String> extensions) {
        requireNonNull(extensions, "extensions");
        for (String extension : extensions) {
            checkArgument(!extension.isEmpty(), "extension is empty");
            checkArgument(extension.charAt(0) != '.', "extension: %s (expected: without a dot)", extension);
        }
        if (fallbackFileExtensions == null) {
            fallbackFileExtensions = ImmutableList.builder();
        }
        fallbackFileExtensions.addAll(extensions);
        return this;
    }
Adds the file extensions to be considered when resolving file names.
This method allows specifying alternative file names by appending the provided extensions
to the requested file name if the initially requested resource is not found.

<p>For instance, if {@code "/index"} is requested and {@code "html"} is an added extension,
{@link FileService} will attempt to serve {@code "/index.html"} if {@code "/index"} is not found.
core/src/main/java/com/linecorp/armeria/server/file/FileServiceBuilder.java