Azure Blob List "Value for one of the query parameters specified in the request URI is invalid" when using API Management service - azure

Question

I'm using API Management service to make a simple interface between a c# desktop application and a Azure Blob storage account.
API Management service is providing the authentication via a Subscription Key. This works fine for downloading blobs.
I also want to list the contents of an individual container / folder (prefix) using the same method.
I've tested the following HTTP call outside the API Management service, it works correctly.
https://<account>.blob.core.windows.net/files?restype=directory&comp=list&prefix=globalpackages/1.0
When I use this in API Management service, I'm returned an error:
<Error>
<Code>InvalidQueryParameterValue</Code>
<Message>Value for one of the query parameters specified in the request URI is invalid.
RequestId:917b0aa8-101e-006b-0709-8e85b0000000
Time:2022-07-02T11:47:29.0834806Z</Message>
<QueryParameterName>comp</QueryParameterName>
<QueryParameterValue>list</QueryParameterValue>
<Reason />
</Error>
My processing rules:
<policies>
<inbound>
<base />
<set-header name="Ocp-Apim-Subscription-Key" exists-action="delete" />
<set-header name="Sec-Fetch-Site" exists-action="delete" />
<set-header name="Sec-Fetch-Mode" exists-action="delete" />
<set-header name="Sec-Fetch-Dest" exists-action="delete" />
<set-header name="Accept" exists-action="delete" />
<set-header name="Accept-Encoding" exists-action="delete" />
<set-header name="Referer" exists-action="delete" />
<set-header name="X-Forwarded-For" exists-action="delete" />
<set-header name="x-ms-version" exists-action="override">
<value>#{string version = "2017-11-09"; return version;}</value>
</set-header>
<set-backend-service base-url="#{
return String.Format("https://<storageaccount>.blob.core.windows.net/files?restype=directory&comp=list&prefix=globalpackages/1.0");
}" />
<authentication-managed-identity resource="https://storage.azure.com/" />
</inbound>
<backend>
<base />
</backend>
<outbound>
<base />
</outbound>
<on-error>
<base />
</on-error>
</policies>
what am I doing wrong?

Answer 1

Based on the documentation available here, the query parameter should be restype=container instead of restype=directory if you need to list blobs inside a blob container.
Once you make this change, you should be able to list the blobs.