Looking at the source, it looks like it happens in Catalyst::Engine::prepare_uploads, which reads

sub prepare_uploads {
    my ( $self, $c ) = @_;

    my $request = $c->request;
    return unless $request->_body;

    my $uploads = $request->_body->upload;
    my $parameters = $request->parameters;
    foreach my $name (keys %$uploads) {
        my $files = $uploads->{$name};
        my @uploads;
        for my $upload (ref $files eq 'ARRAY' ? @$files : ($files)) {
            my $headers = HTTP::Headers->new( %{ $upload->{headers} } 
+);
            my $u = Catalyst::Request::Upload->new
              (
               size => $upload->{size},
               type => $headers->content_type,
               headers => $headers,
               tempname => $upload->{tempname},
               filename => $upload->{filename},
              );
            push @uploads, $u;
        }
        $request->uploads->{$name} = @uploads > 1 ? \@uploads : $uploa
+ds[0];

        # support access to the filename as a normal param
        my @filenames = map { $_->{filename} } @uploads;
        # append, if there's already params with this name
        if (exists $parameters->{$name}) {
            if (ref $parameters->{$name} eq 'ARRAY') {
                push @{ $parameters->{$name} }, @filenames;
            }
            else {
                $parameters->{$name} = [ $parameters->{$name}, @filena
+mes ];
            }
        }
        else {
            $parameters->{$name} = @filenames > 1 ? \@filenames : $fil
+enames[0];
        }
    }
}
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