UploadEndpoint not going straight to cache store

[Floppy]

In my app, I've mounted an upload endpoint to work with Uppy, and I've pointed it at the cache store:

• routes.rb
• uploader

It's my understanding that this should upload directly to the Shrine cache store (at Rails.root + tmp/shrine). However, it seems to be initially storing the file in /tmp, and then copying it over once the upload is complete, but before Shrine can respond back to the browser. This means I get a bit delay once the upload hits 100%.

Anyone got any clues on why it's still writing temporary RackUploadedFile* into /tmp ?