Drop zone to replace main file takes to much space. It could be used to show a bigger preview.
Look into mockups attached or look into https://projects.invisionapp.com/freehand/document/3D8jO6dgO
In terms of main file replacement, I would still consider 2 possible behaviors:
A) Action (icon attached) that will reveal a pop up with extended options to upload new file. The icon, if possible, should be ordered right before the remove blob icon.
B) Have a droparea on top of the image-viewer, triggered when a file is dragged on top of it.
Note: the 'file' label on top of the dialog, would be a variable that would always assume the name of the file that is being replaced, to provide more certainity to the user of what he is actually replacing.