405 Method Not Allowed on Attachment Download

Using the Rest API and authenticating via OAuth 1.0a token in the the authorization header, I get an 405 Method Not Allowed. Sensitive information redacted, of course.

Attachment URL: https://instance.atlassian.net/secure/attachment/xxxxx/filename.ext

The raw request from Fiddler is included below. Oddly enough, this request only succeeds when the proxy is set to Fiddler and Fiddler’s decrypt HTTPS connections is enabled with Fiddler’s own SSL certs installed. Why is this?

GET https://justenough.atlassian.net/secure/attachment/53483/image-2018-07-23-12-19-41-589.png HTTP/1.1
Accept: application/json, application/xml, text/json, text/javascript, text/xml
Authorization: OAuth oauth_consumer_key=“xxx”,oauth_nonce=“xxx”,oauth_signature=“xxx”,oauth_signature_method=“RSA-SHA1”,oauth_timestamp=“1532382483”,oauth_token=“xxx”,oauth_version=“1.0”
Connection: Keep-Alive
Accept-Encoding: gzip, deflate
Host: justenough.atlassian.net