Here's an interpreter script that does what you want. The file names are expected from the command line, for instance from a shell glob.

#!/usr/bin/perl

use Image::Magick;
for (@ARGV) {
    my $p = Image::Magick->new();
    $p->Read($_);
    s/\.[^.]+$/.jpeg/;
    $p->Write($_);
}
[download]

After Compline,
Zaxo