Insert/Edit image upload overwrites image with same name without warning

Description

Steps to reproduce:
1. Create a page with a xhtmlstring property.
2. In the TinyMCE editor, click on "Insert"->"Image," choose the Upload option and upload an image.
3. Notice that an image has been created in the "For This Page" folder.
4. Upload a different image with the same name as in step 1.

Expected:
Should show a warning message that you're about to overwrite an image.

Actual:
Image is overwritten.