Some time ago, Reoto asked a very nice question on Black Storm forum:
Can someone fix the .dll (.net) pe header to MS DOS?
How can I do that?
If you know about protecting .net files for Android, please help me.
I have another question.
Can I fix dnspy to resolve .dll pe header isn't .net?
Obviously, English is not author's first language but it seemed like an interesting problem, so I decided to look into it.
Here is one of the files in question: https://mega.nz/#!0g4VHaIR!KmpQirte4_3lv8MSxyjETiufjFGb-CITpFGrXwxSgGY
TL;DR: Mono loader used by Unity3D accepts invalid PE files. It can be used to break most .NET decompilers. dnlib and tools based on dnlib (dnSpy, de4dot) were updated on 20-Apr-2018 but the rest of the tools still can't handle such files.