dbus-switchboard currently only handles files with file extensions. The simple solution here is to download files and add extensions, then use the file manager to open the files with dbus-switchboard.
The more complex, but far better, solution is to figure out how to pass the detected mime type to dbus-switchboard and use that to determine the handler, rather than the file extension.
Exec=evince StartupNotify=true Terminal=false Type=Application Icon=evince X-Osso-Service=org.gnome.evince.ApplicationService X-Osso-Type=application/x-executable MimeType=application/pdf;application/x-bzpdf;application/x-gzpdf;application/x-dvi;application/x-bzdvi;application/x-gzdvi;image/vnd.djvu;image/tiff;application/x-cbr;application/x-cbz;application/x-cb7;image/*;application/vnd.sun.xml.impress;application/vnd.oasis.opendocument.presentation;