This is where I brought up the fact that it was happening with my Mac, also. The reported solution was to use a different browser for Mac, and to use the Shortcuts app for downloads on the iPhone, as it seems to be a result of the discourse server’s reporting of the MIME types, combined with a Safari security feature, from what I understand, anyway.
I would suggest you try to download from the forum with another iPhone web browser, and if that doesn’t help, just use the iPhone/iPad Shortcuts with the settings that @Paulinko suggested in that thread, until the MIME types debacle gets sorted out. Wish I had better news for you. 