I just figured out that the .xlsm needs to be part of the name.
I goofed. That is not the issue. I still can't import certain Excel Files.
It's .xlsx and not .xlsm
It sounds like you figured out your problem, but I've had problems (have problems) where an exported Excel file won't import into FMP until I first open each excel file, in Excel, and save it (again).
Although the other process which created the Excel file is completed, FileMaker doesn't recognize the Excel file for whatever reason. (Bug, probably). So, in my case, when I click the button in FileMaker which is SUPPOSED to import the Excel file, nothing happens at all.
If I then, separately, manually, (sigh), open and save each Excel file (so much for automation). THEN, finally, FileMaker can import the file. Hurray, I guess.
I'm working with the client to just stop using Excel altogether as his export format.