XSLT: A simple way to merge xml files

I have been researching a bit and found a fairly similar question here:
http://forums.tizag.com/showthread.php?p=76699

Here is what I have come up with, this seems to be mostly working except that Firefox is not recognizing it as an xml file even though I have added the xml:output.

File1.xml (note line two, referencing our transformation):

<?xml version="1.0" encoding="ISO-8859-1"?>
<?xml-stylesheet type="text/xsl" href="https://stackoverflow.com/questions/1510688/merge.xsl"?>
<catalog>
  <data>
    <title>Title1</title>
    <description>Description1</description>
    <myid>1</myid>
  </data>

  <data>
    <title>Title2</title>
    <description>Description2</description>
    <myid>2</myid>
  </data>
</catalog>

File2.xml:

<?xml version="1.0" encoding="ISO-8859-1"?>
<catalog>
  <data>
    <author>Author1</author>
    <date>12/34/5678</date>
    <myid>1</myid>
  </data>

  <data>
    <author>Author2</author>
    <date>87/65/4321</date>
    <myid>2</myid>
  </data>
</catalog>

merge.xsl:

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:transform version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" version="1.0" encoding="ISO-8859-1" indent="yes" />
  <xsl:variable name="with" select="'File2.xml'" />

  <xsl:template match="@*|node()">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()" />
    </xsl:copy>
  </xsl:template>

  <xsl:template match="scene">
    <xsl:copy>
      <xsl:apply-templates select="@*|node()" />
      <xsl:variable name="info" select="document($with)/catalog/data[myid=current()/myid]/." />
      <xsl:for-each select="$info/*">
        <xsl:if test="name()!='myid'">
          <xsl:copy-of select="." />
        </xsl:if>
      </xsl:for-each>
    </xsl:copy>
  </xsl:template>
</xsl:transform>

Output xml when viewing File1.xml:

<catalog>
  <data>
    <title>Title1</title>
    <description>Description1</description>
    <myid>1</myid>
    <author>Author1</author>
    <date>12/34/5678</date>
  </data>

  <data>
    <title>Title2</title>
    <description>Description2</description>
    <myid>2</myid>
    <author>Author2</author>
    <date>87/65/4321</date>
  </data>
</catalog>