Is there a list of supported types of files that can be uploaded documented anywhere? It looks like neither .rtf nor .usdz work with the POST API in my testing. They both upload as weird non-image .jpg files.
Here’s the current list:
jpg
jpeg
mp3
m4a
m4v
mov
mp4
png
gif
webp
svg
pdf
ttf
woff
woff2
eot
xml
txt
text
js
json
css
html
htm
md
zip
gz
jxl
mobi
epub
I’ll add .rtf and .usdz today. I’m also going to move this list into an actual help document somewhere.
Awesome sauce, thank you! 
If you don’t mind pinging here when there’s an official document, I’ll reference that in the future.
(And if there was an endpoint to return that list, that would be super cool too 
)
Update: those two new file extensions are now enabled. Here’s the new help page.
Agreed, we should bake this into the API.