Hi datbates,
Thanks for contacting Apple Support Communities!
We understand that you'd like to delete all attachments from Messages that are stored in iCloud.
In order to delete all attachments, you would need to open each message and delete the attachments from the messages themselves. You can learn more about deleting messages from your iPhone here: Use Messages on your iPhone, iPad, or iPod touch
Find attachments from a conversation
1. Open the message.
2. Tap the name of the contact at the top of the message. If you're using iOS 14 or earlier, or iPadOS, tap the Info button 
.
3. You'll see Photos and Links. Touch and hold an image or attachment to show options like Save, Copy, Share, and Delete.
This article explains how to manage the iCloud storage: Manage your iCloud storage
Delete texts and attachments in Messages
When you use Messages in iCloud, all of your text messages and the attachments you send and receive use your iCloud storage. You can make more space available when you delete texts and attachments—like photos, videos, and files—that you don't need anymore.
iPhone or iPad
To delete a message or attachment, follow these steps:
1. In a message conversation, touch and hold the message bubble or attachment that you want to delete.
2. Tap More.
3. Tap Delete 
, then tap Delete Message. If you want to delete all messages in the thread tap Delete All, in the upper-left corner. Then tap Delete Conversation.
To delete multiple attachments from a contact or conversation, follow these steps:
1. In a message conversation, tap the contact's name at the top of the screen.
2. Tap Info .
3. In the Photos section, tap See All.
4. Tap Select, tap the images you'd like to delete.
5. Tap Delete, then tap Delete Attachment to confirm.
Take care!
