Try opening in DNSpy first, and if that doesn't work there's a possible workaround to view in reflector. The first two bytes when viewed in a hex editor should be (some letter)Z, change that to MZ and try reading it ( if it works tho don't forget what the original two bytes were because you'll need to edit those back when finished)
(Edit rechecked where i had the original info, should try using DNSpy either way)