According to this and this, getByIssueId() and getIssueToken() are Deprecated in JIRA 6.3.x and getByFormToken() should be used instead. However, it's not clear how this method should be used.

Apart from that, formToken is also not available in a Transition screen where Attachment field is available. This makes the method TemporaryAttachmentsMonitor.getByFormToken() not able to work.

This response is received from F12 Developer tools -> Network tab:

Request URL:https://andynguyen.vn/jira/rest/internal/1.0/AttachTemporaryFile?filename=addapplink.png&size=34830&atl_token=BJWG-7248-IGK8-8AG3%7C240c0ccc8ba46cff23bc6019fc527c6571b238d1%7Clin&formToken=null&issueId=10100

From F12 Developer tools -> Elements tab, there's no <input type="hidden" name="formToken" for the corresponding form.

This is not the case with the Attach button, whereby the same input tag is available and the following response is received:

Request URL:https://andynguyen.vn/jira/rest/internal/1.0/AttachTemporaryFile?filename=addapplink.png&size=34830&atl_token=BJWG-7248-IGK8-8AG3%7C240c0ccc8ba46cff23bc6019fc527c6571b238d1%7Clin&formToken=0cd590af3baad807b2cfa6c9425905b859f92323&issueId=10100

Related Question: Can't get temporary attachments from transition screen.