Unfortunately, no. While some iOS functions offer a 'Select All' option, there isn't one for message attachments. There is, however, an option to delete all of your iMessages in one thread at one time. All you need to do is long-press one of the texts within a conversation, tap 'More,' then tap 'Delete All.' You can also delete all iMessages on your phone in the text messaging app. Open the app and tap 'Edit' in the upper left corner. Then, tap 'Delete All.' This will remove all of your texts, attachments, and conversations.