Permissions for editing articles - Uploading files/images/videos

I've come across some odd behaviour when trying to set some permissions for editing articles within groups.

I have a subset of staff members to whom I'm trying to give the permissions to edit the articles across a number of different groups. I've created a site role for them, placed them in that role and have allowed that site role only the permissions to edit the articles within the group. I've not allowed them any other group permissions as I don't want them messing around with anything else or creating new articles within the group, only editing those that are already there.

However, when they edit the article they can add additional text and hyperlinks etc, they can even insert polls and emojis and everything else under the 'insert' menu of the editing toolbar EXCEPT insert a file, video or image. 

When they try to insert a file etc, there is no upload link for them to select and browse to find the file/image/video to upload. BUT if I give them the permissions to 'create articles' then the link with then appear if they try to edit the existing ones. However, I don't want them to be able to create any new articles in the groups, just edit the existing ones.

Is there something not quite right with the coding within the Edit permission's for articles? For blogs and forum posts permissions the ability to upload files is listed as an entirely separate permission but not for Articles.

Can someone shed some light?