TriggerOnAttachment

AttachmentTrigger.cls

trigger AttachmentTrigger on Attachment (before insert) {
	set<String> setExtNotAllowed = new set<String> {'exe','dll'};
	for (Attachment attachment :Trigger.new) {
		String strFilename = attachment.Name.toLowerCase();
		List<String> parts = strFilename.splitByCharacterType();
			if(setExtNotAllowed.Contains(parts[parts.size()-1])) {
				attachment.addError('File with extension exe or dll could not be attached!!');
			}
	}
}