Sharepoint Content Query Webpart Condition based left icon display.

1. Open SharePoint Designer, Go to "Style Library/XSL Style Sheets/ItemStyle.xsl"
2.  Add the below code

       <div class="link-item">
              <xsl:call-template name="OuterTemplate.CallPresenceStatusIconTemplate"/>                     
                <xsl:variable name="extension">
                    <xsl:call-template name="get-file-extension">
                        <xsl:with-param name="path" select="@LinkUrl" />
                    </xsl:call-template>
                </xsl:variable>   
                <xsl:choose>
                    <xsl:when test="$extension = 'docx'">
                        <!-- add your logic -->
                         <img src="/images/icons/blogpost.png"></img>
                    </xsl:when>
                     <xsl:when test="$extension = 'xlsx'">
                        <!-- add your logic -->
                         <img src="/images/icons/Docment.png"></img>             
                    </xsl:when>
                </xsl:choose>        
               
                <a href="{$SafeLinkUrl}" title="{@LinkToolTip}">
                  <xsl:if test="$ItemsHaveStreams = 'True'">
                    <xsl:attribute name="onclick">
                      <xsl:value-of select="@OnClickForWebRendering"/>
                    </xsl:attribute>
                  </xsl:if>
                  <xsl:if test="$ItemsHaveStreams != 'True' and @OpenInNewWindow = 'True'">
                    <xsl:attribute name="onclick">
                      <xsl:value-of disable-output-escaping="yes" select="$OnClickTargetAttribute"/>
                    </xsl:attribute>
                  </xsl:if>                
                  <xsl:value-of select="$DisplayTitle"/>
                </a>               
                <div class="description">
                    <xsl:value-of select="@Description" />
                </div>
            </div>

_________________________
Add get file extension template

   <xsl:template name="get-file-extension">
        <xsl:param name="path"/>
        <xsl:choose>
            <xsl:when test="contains($path, '/')">
                <xsl:call-template name="get-file-extension">
                    <xsl:with-param name="path" select="substring-after($path, '/')"/>
                </xsl:call-template>
            </xsl:when>
            <xsl:when test="contains($path, '.')">
                <xsl:call-template name="get-file-extension">
                    <xsl:with-param name="path" select="substring-after($path, '.')"/>
                </xsl:call-template>
            </xsl:when>
            <xsl:otherwise>
                <xsl:value-of select="$path"/>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>