The method which Tim describes below is now the only way to import an assembly into a single part file. See attached sample.
This is mentioned in the Help here:
"When you import a file, Inventor automatically detects whether the imported file is a part or assembly and creates a new document accordingly. You can then preview and select imported items. To import a 3rd party assembly file as a part, you must first create or have a part file open, and then import the 3rd party assembly file into the part file."
(Yes I know you shouldn't have to read the Help for something that has worked forever....)
However, after reviewing some of the related Help for importing assemblies from here, I see that it does require some attention; it still implies the use of the option:
- "Multi-body part imports an assembly as solid bodies in a single part."
I'll request that is reviewed/enhanced.
Some of the further complication here, (in addition to the changed workflow from prior releases), is as Tim mentions, a defect which is planned to be addressed in SP1. ie when using the Import command in a part, the .step & .stp file extensions are not explicitly listed & are not available when using the "All Models" filter, so only "All Files" will show them for selection.
The support for importing assemblies into parts & drag/drop workflows were discussed/reviewed as part of the 2016 design/implementation but could not be supported based on some of the changes required for other aspects of the whole AnyCAD associative architecture.....(hence, my earlier comment about the frequent loss of elegance/sophistication when UI simplifications are performed as part of another initiative).
Yes, step is one of the most ubiquitous neutral file formats in use; the good news is that it is receiving considerable focus for the next release of Inventor. If you would like to be part of further NDA based discussions, (& get up/close & personal with the Designers/Developers), then you can apply for Inventor Alpha/Beta membership on https://beta.autodesk.com using this one-time application link:
https://bit.ly/InventorBeta
(Sam, you already have access to the Alpha/Beta project for Inventor so no need to reapply with that link).
Hope this helps explain, (even if it doesn't really help with the frustration right now).
Chris

Chris Mitchell
PDMS Customer Engagment Team
Autodesk, Inc.