Are you saying you're seeing a problem with files attached on desktop and synced to mobile? This must be a different root cause because the message you replied to was about an issue affecting only Android.
It does not seems to matter where attachment is made mobile or desktop. It refuses to open on android client with No app associated with this mime type error.
So far I have seen this behaviour with pdf,docx, and epub files. But I am doubting that this a generic problem with all attachments types requiring external applications. I am saying this because picture attachment which are handled by Joplin natively seems to work fine.
Hope this clarifies the situation.
That's another issue. I believe it's been fixed in the latest pre-release. Or if not will be included in the next one.
Thanks new version 2.6.4 seems to fix it.