in more recent versions of K9-Mail, there is no more option in the settings to select the Download folder (but K9 used to have that, IIRC. Is it gone or is this a bug?).
When I press the Download button next to an attached file, the file does not end up being saved in the “Downloads” folder of the device (neither internal storage nor external SD card Downloads). Where does K9-Mail save attachments to nowadays?
Additional question: When I have a calendar invitation (ics file) and do not download it by the download button, but click on it directly, my calendar app (business calendar) opens up for importing, but it cannot find the file at the preset location. Where is the file temporarily stored in this other use case? (is it a limitation of K9, or more likely business calendar or Android in general that I cannot hand over the file from K9 to calendar?)