Serve all files with a Content-Disposition of 'attachment' via WebDAV

As an additional security hardening it's sensible to serve these files with a Content-Disposition of 'attachment'. Currently they are served 'inline' and get a "secure mimetype" assigned in case of potential dangerous files.

To test this change ensure that:

- [ ] Syncing with the Desktop client still works
- [ ] Syncing with the Android client still works
- [ ] Syncing with the iOS client still works

I verified that the 1.8 OS X and iOS client still work with this change.