Match an element with an xsi:type attribute

I have an XML file which contains the following:

<contents id="MLC1" name="Requirement1" uri="C:\abc.txt" xsi:type="requirement:Requirement" type="">
<contents id="GO1" name="Goal1" uri="C:\abc.txt" xsi:type="goal:Goal">

I am trying to match all the element within my XML file, which has the attribute xsi:type="requirement:Requirement" so that I can add a new attribute called "label" to it. This is my stylesheet:

<xsl:template match="//contents[@type='requirement:Requirement']">
    <contents>
        <xsl:attribute name="label">
            <xsl:value-of select="@name"/>
        </xsl:attribute>        
        <xsl:apply-templates select="node()|@*"/>
    </contents>
</xsl:template>

I have already declared xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" in my stylesheet, but it seems that it is unable to match anything. I am suspecting it is because there is also another attribute "type" in the original xml which does not have the xsi namespace. Does anyone has any suggestion what should I use to match this element correctly?