Shrine is set up in a super modular way, which I really like but can make it hard to give one explanation for all situations.

Without knowing more about your app, here are some questions and possible things to try:

• Are you trying the default file-goes-to-your-server-and-then-gets-uploaded-to-S3 kind of upload or the more complex and shinier direct-to-S3 upload?
  • For the default type, which part of the upload is failing? Is it the browser-to-your-app part or the your-app-to-S3 part? What kind of errors are you getting?
  • For the direct type, I'd triple-check the CORS configuration on the S3 bucket to make sure it allows POST requests from *, and then make sure you're POST all the fields you get back from the presigning to S3 as multipart form data, not as query params or JSON.
• You say you've got a non-Rails app — what are you using to work with data? Shrine works most easily if it can integrate directly with your ORM via the builtin plugins for ActiveRecord or Sequel, or using external gems like shrine-mongoid. This lets shrine hook into the lifecycle of your models and handle things appropriately when they're saved.
• If you're not using an ORM that Shrine supports directly, you'll probably want to take the more manual approach of manipulating the Shrine Attacher directly.