Comment # 2 on bug 1099766 from Wolfgang Bauer

The problem is that Falkon displays the .ymp file as text (when you click on a
"1-click installation" button on https://software.opensuse.org/search/ )
instead of actually installing the package...

See also bug#836059 about Chromium.

I just tried (Falkon 3.0.0 on Leap 42.3 with Qt 5.11.1) and this is still the
case.

Chromium has this patch that is supposed to fix it:
https://build.opensuse.org/package/view_file/network:chromium/chromium/exclude_ymp.patch?expand=1
Maybe it's possible to "port" it to QtWebEngine?

Although I'm not sure that's sufficient.
Chromium also has this in the shipped master_preferences file:
  "download": {
     "extensions_to_open": "ymp"
  },