Inserting "false image" shows "Page Not Found Error"

Steps to reproduce:

1-) Rename a .pdf file (ex: book.pdf) into a .gif file (ex:. book.gif)
2-) Try to insert the .gif file in a Confluence page as an image (using "Insert Image" pop-up window)
3-) An "Page Not Found" error page is shown (see attached screenshot)

It should show an "Invalid File" message or something similar.