Added new JB/file_exists helper.

_includes/JB/file_exists

@@ -0,0 +1,26 @@
+{% comment %}<!--
+  param:  file = "/example/file.png"
+  return: file_exists_result = true
+
+  examples:
+    {% include JB/file_exists file="/404.html" %}
+    {% if file_exists_result %}Found "/404.html"!{% else %}Did not find "/404.html".{% endif %}
+
+    {% assign filename = "/405.html" %}
+    {% include JB/file_exists file=filename %}
+    {% if file_exists_result %}Found "{{ filename }}"!{% else %}Did not find "{{ filename }}".{% endif %}
+
+  NOTE: the BREAK statement in the FOR loop assumes Liquid >= 2.5.0
+
+-->{% endcomment %}
+
+{% assign file_exists_result = false %}
+
+{% if include.file %}
+	{% for static_file in site.static_files %}
+		{% if static_file.path == include.file %}
+			{% assign file_exists_result = true %}
+			{% break %}
+		{% endif %}
+	{% endfor %}
+{% endif %}