To test, send this image in a Yahoo email:

https://gallery.eocampaign1.com/cf75d55f-4b21-11e4-a4c4-0afa0141605b%2F1542764377025-test.jpg …

The Yahoo proxy rewrites to:

https://ecp.yusercontent.com/mail?url=https%3A%2F%2Fgallery.eocampaign1.com%2Fcf75d55f-4b21-11e4-a4c4-0afa0141605b%252F1542764377025-test.jpg&t=1542764441&ymreqid=82730191-abe4-9838-1c08-fa0021012e00&sig=aDcKKOF6X3K8ZpfAcGBUtA--~C

Here is a random image on the internet that also exhibits the issue:

https://jonnyjordan.com/wp-content/uploads/2018/03/magento-image-upload-button-missing.jpg