K-9 Mail doesn’t write a file to the download folder when you tap the attachment in the message view screen. K-9 Mail will try to launch an app using a URI that references the attachment data in K-9 Mail’s app-private storage. The URI typically looks something like this:
content: is a URI scheme specific to Android. It allows the receiving app to query further information about the content, e.g. the display name (file name) and the size.
It looks like NeoReader doesn’t support that last step. Try raising the issue with your device vendor and have them point their developers at OpenableColumns | Android Developers
Things will probably work fine if you manually save the attachment to the download folder (tap the button on the right end of the attachment box) and then open the file from there.