... causes banshee to use a _huge_ 512x512 icon instead. Of course, creating the two required icons in ~/.local/share/icons works around this issue, but having the icon with the desired size in the default theme would be better (size 128x128 and 16x16 are needed for an external usb media player).
Hmm. I see: /usr/share/icons/Adwaita/16x16/devices/multimedia-player.png /usr/share/icons/Adwaita/22x22/devices/multimedia-player.png /usr/share/icons/Adwaita/24x24/devices/multimedia-player.png /usr/share/icons/Adwaita/32x32/devices/multimedia-player.png /usr/share/icons/Adwaita/48x48/devices/multimedia-player.png /usr/share/icons/Adwaita/512x512/devices/multimedia-player.png All of those come from adwaita-icon-theme-3.24.0-2.fc26.noarch It looks like 128x128 is the only missing size there, but the adwaita theme does not provide _any_ icons in that size. Banshee doesn't include any "multimedia-player.png" icons. I'm not exactly sure what to do here. If you think adwaita should include 128x128 versions of their icons, then that is a bug against adwaita-icon-theme. If there is a missing size for a banshee included icon, let me know the specific filename(s) and I'll resize and include.
What surprised me is that the behavior is clearly new in Fedora 26, and after strace-ing banshee on a Fedora 25, it appears that it uses a downscaled 256x256 multimedia-player.png icon, that is provided in adwaita-icon-theme-3.22.0, but has been dropped in 3.24 (this icon has been redesigned): https://koji.fedoraproject.org/koji/fileinfo?rpmID=8105146&filename=/usr/share/icons/Adwaita/256x256/devices/multimedia-player.png We could expect that the actual 512x512 icon would be downscaled to the requested size as well, but this is not the case, because icons of this size (section [512x512/devices] in file index.theme) are not given the possibility to be used at other sizes. To summarize, yes, I think this is a bug in adwaita-icon-theme 3.24. I'll fill a bug in gnome bugzilla for it.
adwaita-icon-theme has been fixed upstream: https://git.gnome.org/browse/adwaita-icon-theme/commit/?id=824af3c19333c1e310dbb6818398ead290ddb11d
I'm going to close this as NOTABUG, since the icon will presumably show up in the adwaita-icon-theme package.