Download attachment wich WLAN always

Hallo, I live K9 and hope. But I miss an General option, that Mails ans attachments are downloaded, if system is logged in into a WLAN.
Could you implement this? Regards

Why don’t you use Android’s own tools for that?

Settings → Network and Internet → Data Saver

Just configure it in a way that any app you don’t want to use mobile data to be blocked.

Hi, this is not, what I’m looking for.
K9 should be able to sync at any time and all other apps too. But k9 should download in background full mail including attachments like pictures, docs, pdf …, IF I’m connected to a WLAN.