I see what you mean. Ok -- I don't have the 6.1 source code handy right now but I looked it up in the 6.2 codebase -- and will go out on a limb here and assume that they're the same, or similar at least.
There is a file in TOMCAT_HOME/webapps/ROOT/html/portlet/document_library/file_entry_columns.jspf that you can hook. In this file thre is a section --
<c:when test="<%= fileEntryColumn.equals("name") %>">
<liferay-ui:search-container-column-text href="<%= rowHREF %>" index="<%= j %>" name="<%= fileEntryColumn %>">
<%
String thumbnailSrc = DLUtil.getThumbnailSrc(fileEntry, latestFileVersion, fileShortcut, themeDisplay);
%>
<span class="entry-thumbnail">
<img alt="<liferay-ui:message key=" thumbnail">" class="thumbnail" src="<%= thumbnailSrc %>" />
<c:if test="<%= fileShortcut != null %>">
<img alt="<liferay-ui:message key=" shortcut">" class="shortcut-icon" src="<%= themeDisplay.getPathThemeImages() %>/file_system/large/overlay_link.png" />
</c:if>
<c:if test="<%= fileEntry.isCheckedOut() %>">
<img alt="<liferay-ui:message key=" locked">" class="locked-icon" src="<%= themeDisplay.getPathThemeImages() %>/file_system/large/overlay_lock.png" />
</c:if>
<c:if test="<%= ((latestFileVersionStatus != WorkflowConstants.STATUS_ANY) && (latestFileVersionStatus != WorkflowConstants.STATUS_APPROVED)) %>">
<aui:workflow-status showIcon="<%= false %>" showLabel="<%= false %>" status="<%= latestFileVersionStatus %>" />
</c:if>
</span>
<div class="entry-metadata">
<span class="entry-title"><%= HtmlUtil.escape(latestFileVersion.getTitle()) %></span>
<c:if test="<%= Validator.isNotNull(latestFileVersion.getDescription()) %>">
<div class="entry-description">
<%= HtmlUtil.escape(latestFileVersion.getDescription()) %>
</div>
</c:if>
</div>
</liferay-ui:search-container-column-text>
</c:when>
-- you can see in this block that there is an <img/> tag that set's the thubnail source calculated right above it with --
<%
String thumbnailSrc = DLUtil.getThumbnailSrc(fileEntry, latestFileVersion, fileShortcut, themeDisplay);
%>
.. and I guess it can't calculate anything for your html files. So you have two options I guess. You could hook the JSP and add logic to detect the absence and then hook into your own util to generate one, OR, what I would guess is a better solution, would be to create your own document type that contains a field that allows you to specify an image when you upload the HTML file. The second option would require you upfront to generate the image to have it handy for when you create the content item -- but I would say that it is a more appropriate use of the system (over the JSP hook and mangling the display logic).
