I am curious about how I could fill the hashes dynamically

That largely depends on where you get the info from.  Let's say you read a list of image paths from a file  (__DATA__ here):

#!/usr/bin/perl -w
use strict;

my @imgs;
while (my $path = <DATA>) {
    chomp $path;
    my ($fullname, $name, $ext) = $path =~ m#(([^/]+)\.(\w+))$#;
    my $type = "image/$ext";
    $type =~ s/jpg$/jpeg/;
    push @imgs, {
        Type        => $type,
        ID          => $name,
        Path        => $path,
        Filename    => $fullname,
        Disposition => 'attachment',
    };
}

use Data::Dumper;
print Dumper \@imgs;

__DATA__
/images/logo.gif
/images/footer.gif
/images/whatever.gif
[download]

produces:

$VAR1 = [
          {
            'ID' => 'logo',
            'Disposition' => 'attachment',
            'Type' => 'image/gif',
            'Filename' => 'logo.gif',
            'Path' => '/images/logo.gif'
          },
          {
            'ID' => 'footer',
            'Disposition' => 'attachment',
            'Type' => 'image/gif',
            'Filename' => 'footer.gif',
            'Path' => '/images/footer.gif'
          },
          {
            'ID' => 'whatever',
            'Disposition' => 'attachment',
            'Type' => 'image/jpeg',
            'Filename' => 'whatever.jpg',
            'Path' => '/images/whatever.jpg'
          }
        ];
[download]