Thanks for reporting back that it works for you. I have a solution, though I think that there is a problem hidden: If I change my email address to a gmail address (which I have but hardly ever use), I receive the emails, I don't know if this is also true for other recipients. As I own my standard emailing domain which is hosted by a major provider, ldschurch.org is whitelisted. The emails do not appear in the spam folder either. I don't find any traces of them in my non-gmail folder, though I don't have access to the server logs.
I tried it with various browsers and file types: jpg, pdf; browsers: chrome and edge(Windows only); operating systems windows 10 and mac. File sizes is a couple of KB, PDF had something like 146 KB, JPEG 506. I don't even get the typical verification email that I receive if I don't use an attachment.
If this behavior is true and some provider settings do not allow receiving the emails with attachments independent of their personal settings, then we cannot rely on the recipient even getting this email. For me gmail might be an option, but that is definitively not true for all members. In short: it seems to be a problem on the receiver side with spam protection or something that is deeply hidden in the MTA, usually not to be influenced by a user.