There is a string in the forum config that says: "allowed attachments: 'jpg,zip..'.
Adding jpeg to this shouldn't be hard.
To be precise: attachmentCheckExtensions
if (!empty($modSettings['attachmentCheckExtensions']))
{
if (!in_array(strtolower(substr(strrchr($_FILES['attachment']['name'][$n], '.'), 1)), explode(',', strtolower($modSettings['attachmentExtensions']))))
fatal_error($_FILES['attachment']['name'][$n] . '.<br />' . $txt['cant_upload_type'] . ' ' . $modSettings['attachmentExtensions'] . '.', false);
}
Defaults seem to be: doc,gif,jpg,mpg,pdf,png,txt,zip
I've not seen an mpg in ages.