@Nullable
    protected final MediaType contentType() {
        return contentType;
    }
Returns the {@link MediaType} of the file, which will be used for setting the {@code "content-type"}
header.

@return the {@link MediaType} of the file, or {@code null} if the {@code "content-type"} header will not
        be set automatically.
core/src/main/java/com/linecorp/armeria/server/file/AbstractHttpFile.java