I have tested this in both Firefox and Chrome browsers, resized windows and confirmed working in the classic osTicket. Also tested with several different image files and image file types with same result. Tested with non image files and both drag and drop and choosing the file keeps the attachment.
I decided to test as an agent instead of user and found that the upload works when attempting to upload in the agent panel. I also noticed it shows a file preview when in the agent panel but not from the user side of things.