You can use logical operators on the attributes.

#!/usr/bin/env perl
use warnings;
use strict;
use feature qw{ say };
use XML::LibXML;

my $dom = 'XML::LibXML'->load_xml(location => 'imgfile.xml');
say 'XML Version is: ', $dom->version;
say 'Document encoding is: ', $dom->encoding;

my $query = '(@name="Name" and @class="FILENAME") or (@name="Size")';
my $xpath = '/files/file[@name="Image"]/item/field['.$query.']';

for my $node ( $dom->findnodes($xpath) )
{
  print "\n";
  say 'Name  : ', $node->getAttribute('name');
  say 'Class : ', $node->getAttribute('class');
  say 'Value : ', $node->getAttribute('value');
  
}
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