angular-file-saver | AngularJS service that provides cross | Frontend Framework library

First off: I'm new to Angular so the terms here might be wrong. I hope I can get the understanding through.

I'm consuming an API. Endpoint A exposes Tickets; these Tickets hold an array (0-*) of Attachments. Those Attachments are not complete; in the data model they contain the base64 string of the content of the underlying file but to reduce general load the Attachments are only complete when fetched from Endpoint B. But in order to fetch an Attachment from Endpoint B I need the Attachment's id which I can only get from Endpoint A.

What I need to happen is that I fetch attachment.id from Endpoint A, use that id to fetch the approriate Attachment from Endpoint B, and replace Attachments in the result from Endpoint A with the result from Endpoint B. However since the Attachments are retreived as Observable I can't just simply assign the value.

The following snippet is one of several attempts at solving this and is able to get the Attachments from the API but they aren't assigned the way I wanted.