Instead of ~media({MediaArchive:266ad17b-d5ca-4f3b-b6e2-8733ebc75102})
it should be
~/media(MediaArchive:266ad17b-d5ca-4f3b-b6e2-8733ebc75102)
No {} brackets and starts with ~/
file: "<xsl:value-of select="concat('~/media(',$videofile/@KeyPath,')')"