Anonymous users cannot add attachments to issues

We have a project in our JIRA where anonymous users can create issues. However, when someone tries to add an attachment, the upload bar in the Create Issue screen turns red and it says "Unauthorized". When inspecting the requests in Chrome's network tab it shows we're getting a 401 response to a GET request to /rest/auth/1.

• Group (Anyone) has the following permissions in the project's scheme:
  • Browse Project
  • Create Issue
  • Create Attachment
• Issue Security Scheme is disabled
• Reporter field is optional
• Accept remote API calls is enabled

What else could we be missing?