When there are no allowed attachment extension groups for either a post or a PM, there are no 'plupload filters'. Meaning that plupload will set up as normal. However, whenever you try to upload any file, regardless of file extension, size or dimensions, nothing happens. This is because no file is allowed.

So when there are no filters, read: allowed extensions, the attachment tab shouldn't be displayed and plupload should not be set up.

Additionally there should be a notice message in the ACP's Attachment Settings informing the administrators that are attachments in general are enabled, but no extension groups are allowed.