Epichrome: custom rule for Gmail attachments

I use Epichrome to create a site-specific browser for Gmail. Clicking attachment icons in Gmail causes a new window to open in my system's default brower (Chrome), which I'd like to avoid. Epichrome Helper's "Custom Rules" should help here, but I can't figure out the right patte to use.

Here are the two pattes I'm using:

https://mail.google.com/mail/.*
https://mail-attachment.*

for both rules, set "And would open in:" to "Either" and "Then open it with:" to "This app". But clicking attachments still causes them to open in my system's default browser.

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