Hi Themers,
In MDL-50690 I have solved an issue whereby if you only have an svg version of an image in your theme and an old style browser accesses your installation after an upgrade / purge all caches then the image will still be seen by supporting browsers. The update has been integrated into the next release and will be out shortly on official channels. If you update directly from Moodle's GitHub account then the fix will be there. See Git_for_Administrators.
I would like to thank all those involved in the issue.
If you find this post useful, please mark it so.
Cheers,
Gareth