I have been using the iOS 11 native mail client since the first beta to access multiple Microsoft Exchange accounts (on-prem Windows 2016/Exchange and O365) without any issues. In fact, I use it more than my desktop email. I send out files with multiple attachments, and at least one of my accounts has an image in the email signature.
However, it is possible that all of my exchange end-points did not implement http/2 (I don't have visibility into those settings). It would not be the first time Microsoft rolled out change without properly publishing it in beta's, especially if the change impacts potential security holes (the Microsoft AD and SharePoint APIs have a long, frustrating history of these types of changes).
I would be curious if other iOS email apps that use the iOS Exchange account settings (e.g. Spark/Airmail) are having an issue as well. Outlook for iOS does NOT use the iOS exchange settings, which drives my IT people nuts because for every device I have 2 unique mobile device IDs for exchange (one for iOS apps and one for Outlook for iOS). During the iOS 11 beta, I have run both Spark and Airmail without any issues against Exchange, but again, I do not know if any of my exchange end-points implemented the http/2 change.