Postman is a powerful GUI platform for API development and testing on different systems.
A Postman collection is a group of individual requests. The collection here contains a small list of requests that demonstrate the basic usage of the imagej-server APIs.
Once installed Postman, import the collection and environment JSON files by clicking the Import button at the top left corner. Select imagej_server as the environment. Then go to the Body tab of the upload_img request, and choose a file for upload. You can press ctrl + s to save this request so that you don't have to repeat this process next time.
Then launch an imagej-server instance. Try clicking on different requests and hit the Send button to see the results. Notice that some requests are chained so that one needs to be run before another. For example, you need to run upload_img before retrieve_img or create_img_from_img.
The collection is actually setup as an integration test suit. You can click the Runner button next to the Import, and hit Start Test on the imagej_server collection with imagej_server as the environment. This will not show results from individual requests, but will show some statistics and whether they pass tests.
After getting more familiar with Postman, you can try to create your own requests. Using global variables in the Tests tab will be very helpful for handling random object IDs. Checkout and this document for more details.