Why can't I delete files or photos in Files?
When viewing files and photos in Files, it seems like you can't delete them.
However, Files only shows files or photos attached to messages in your mailbox. To remove the files or photos, you'll need to go to your Inbox or Sent Items folder and delete the message they're attached to.
How to remove files or photos
To remove a file or photo from Files, you must delete the email it's attached to:
For a file or photo in All, find the corresponding email in your mailbox and delete it.
For a file or photo in Sent files, go to your Sent Items folder, find the corresponding email and delete it.
After deleting the email, you also need to delete it from your Deleted Items. Go to your Deleted Items folder, select the item to remove, and select Delete.
Note: If the email is important, save the images to your computer or OneDrive, forward the email to yourself without the images, then delete the original email.