Google App Engine & HTML file in Filter mapping

Google App Engine does not support HTML file in Filter mapping.When I specified my HTML file in filter’s URL-Pattern the filter does not invoke my HTML file.Below is my filter with html file in filter’s URL-Pattern

<filter-mapping>
<filter-name>OSISFilter</filter-name>
<url-pattern>/main1.html</url-pattern>
</filter-mapping>

Then I ended up changing my HTML file into JSP, now my filter works fine with JSP file.Below is my filter with jsp file in filter’s URL-Pattern

<filter-mapping>
<filter-name>OSISFilter</filter-name>
<url-pattern>/main1.jsp</url-pattern>
</filter-mapping>