Fixes for binary files
- Doubly ensure that we don't let the user try to edit a binary file
- Add a `Content-Disposition` header when viewing binary files
Inspired from https://pagure.io/fork/stbuehler/pagure/c/e5263d9daee21f7dde3d3cc02f6d481452f1b019
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>