I have this problem in the Trello api "add attachment" via binary data. It seems that when the binary data is being pass to the card it changes to an unknown file.
I am using a function that will convert HTML to PDF and use its Binary Data output to pass to the request. I already tested it if the file is downloadable in local and it has no problem so far.
This is the structure of the request that I am using.
For reference, I made this Glitch project to demonstrate ways to use the attachments API https://glitch.com/edit/#!/trello-attachments-api?path=README.md%3A1%3A0
Briefly looking at it again leads me to believe that there isn't a way to upload a PDF as Binary Data and have it attach as a .pdf to the card. The API does something a bit wonky and won't hold onto the filtetype as you'd expect.
@bentley that's what I've found too, however this dude reckons he got it working, I haven't had the chance to test yet:
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