Thanks @bridget !
What I was using the JSON file for was storing a Vega plot specification - it can be quite a long object. I was then loading it into a Vega plot, implemented as a custom component along the lines of Embedded interactive visualisations in Anvil using e.g. Vega Lite? . I attempted to store it in Assets because the Javascript snippet in the custom HTML of the component was reading it in directly. But I found that I couldn’t conveniently store it in Assets as I couldn’t edit it (unless I turned it into Javascript instead of JSON).
However since I wrote this question I’ve shifted things around so that the plot specification object is stored on the Python (client-side/Skulpt) side of things and fed in to the component from there. So right now I’m actually storing it as a Python object. I could still conceivably store it as a JSON file and read it in from Python, and I still wouldn’t be sure where is the best place to store that file I suppose!