I think this is a regression. pasting does not work in visual mode, when focus is on the visual mode textarea.

steps to reproduce:

1. Log in to Jira 
2. press "create" to open new issue dialog
3. ensure that visual mode is selected in description
4. copy file from file browser (ctrl-c)
5. click on visual textarea, ensure that cursor is visible in that textarea
6. paste (ctrl+v) image onto visual textarea - only image name is pasted. image should be uploaded and should be inserted and visible in visual textarea

Now it behaves correctly in text mode:

1. switch to text mode
2. click on text mode textarea
3. paste image (ctrl+v) - image is uploaded, visible in uploaded image area and proper wiki markup is generated - this is correct behaviour

Note that pasting (ctrl+v) works when you click on create issue dialog (outside of visual mode textarea)

I've tested it and it worked in some 7.2.x Jira where RTE was opt-in, it didn't work in 7.3.9 and later Jira

It looks like regression