Other people are more qualified to reply, but my first thought is that having multiple files attached to one message could more easily reach the file size limit of your mail provider.
Perhaps one could assume that all email providers allow at least 5 MiB attachments.
DC could calculate the total size of files and warn if they exceed 5 MiB.
Unfortunately, I donāt know if that would mean a lot of development effort.
But it would certainly be a big improvement, especially for sending photos.
However, there is a workaround. It is possible to send multiple attachments with another email client using the DC account. The client needs to support Autocrypt for verified chats and Chatmail accounts. Without Autocrypt, the encryption is temporarily suspended for ānormalā accounts.