There are multiple administrative areas of vBulletin where you can control attachment permissions. Here are the paths to the various areas in the Admin CP:
Attachments > Attachment Permissions where (in the Attachment Permissions Manager) you can control attachments by file extension (for example, jpg or pdf). For more information, see the Managing Attachment Permissions article.
Settings > Options > Message Attachment Options where you can set general options for message attachments (for example, limit the total space taken up to attachments or allow duplicate attached images). For more information, see the Managing Attachment Permissions article.
Usergroups > Usergroup Manager, where you can control downloading and uploading attachments. For more information, see the Adding and Editing A Usergroup article.
Usergroups > Channel Permissions, where (in the Channel Permissions Manager) you can control downloading and uploading attachments, creating attachments, and moderating attachments. For more information, see the Adding and Editing Channel Permissions article.
Channel Management > Channel Permissions, where (in the Channel Permissions Manager) you can control downloading and uploading attachments, creating attachments, and moderating attachments. For more information, see the Adding and Editing Channel Permissions article.