@Nullable
    public static String fileServiceCacheSpec() {
        return FILE_SERVICE_CACHE_SPEC;
    }
Returns the {@linkplain CaffeineSpec Caffeine specification string} of the cache that stores the content
of the {@link HttpFile}s read by a {@link FileService}. This value is used as the default of
{@link FileServiceBuilder#entryCacheSpec(String)}.

<p>The default value of this flag is {@value DefaultFlagsProvider#FILE_SERVICE_CACHE_SPEC}. Specify the
{@code -Dcom.linecorp.armeria.fileServiceCache=<spec>} JVM option to override the default value.
For example, {@code -Dcom.linecorp.armeria.fileServiceCache=maximumSize=1024,expireAfterAccess=600s}.
Also, specify {@code -Dcom.linecorp.armeria.fileServiceCache=off} JVM option to disable it.
core/src/main/java/com/linecorp/armeria/common/Flags.java