support extracted output in HTML template

allows

    ipython nbconvert --to html --ExtractOutput.enabled=True

to work properly without any b64-encoded data.

IPython/nbconvert/templates/html/basic.tpl

@@ -113,7 +113,11 b' unknown type {{ cell.type }}'
 {%- endblock stream_stderr %}
 
 {% block data_svg -%}
+{%- if output.svg_filename %}
+<img src="{{output.svg_filename | posix_path}}"
+{%- else %}
 {{ output.svg }}
+{%- endif %}
 {%- endblock data_svg %}
 
 {% block data_html -%}
@@ -123,7 +127,11 b' unknown type {{ cell.type }}'
 {%- endblock data_html %}
 
 {% block data_png %}
+{%- if output.png_filename %}
+<img src="{{output.png_filename | posix_path}}"
+{%- else %}
 <img src="data:image/png;base64,{{ output.png }}"
+{%- endif %}
 {%- if 'metadata' in output and 'width' in output.metadata.get('png', {}) %}
 width={{output.metadata['png']['width']}}
 {%- endif %}
@@ -134,7 +142,11 b" height={{output.metadata['png']['height']}}"
 {%- endblock data_png %}
 
 {% block data_jpg %}
+{%- if output.jpeg_filename %}
+<img src="{{output.jpeg_filename | posix_path}}"
+{%- else %}
 <img src="data:image/jpeg;base64,{{ output.jpeg }}"
+{%- endif %}
 {%- if 'metadata' in output and 'width' in output.metadata.get('jpeg', {}) %}
 width={{output.metadata['jpeg']['width']}}
 {%- endif %}