You can still do this. It's a few more steps than you used previously.
In the message thread, tap on the picture of the person who sent the message. That reveals a sub-menu. Tap on the info icon. That reveals a comprehensive display, including all documents sent by them. You can locate the file in the documents list and proceed as usual.
The old way to access attachments in messages was:
press and hold until a menu pops up
Select "Share" icon
Choose the FileMaker Go app from the list of icons
You still have to perform those steps, but first perform three non-intuitive steps. They take you from the document you want to act on, to a list of documents, so you can possibly select the wrong one. So much better?