Since I try to have posts for all the adapters I have encountered it can be that this is a post that it not new for all the people already working with BizTalk for a long time. It just wanted to have it on my blog for the sake of completeness;
The FILE adapters requires that the Host Instance account user has FULL Control on the folder it tries to read from.
I was giving a course on security in BizTalk and explained the security modal of BizTalk (like BizTalk operator / BizTalk administrator) and the ability to run Host instances under minimal privileges accounts (BizTalk Application Users member) and had a hard time to explain why the file was not picked up when Read only rights were assigned.
This answer is: because I said so