Cant open downloaded files until copy/duplicate/rename them.

From a mount or two now I can open any downloaded file until I rename a single letter. copy it somewhre else or duplicate it. It says different errors depending of the format. For Excel is this:

Excel cannot open the file because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

Other time it says I dont have privileges to open it ( i checked and the original or duplicated files have same privs ). As soon as I change something on it - it works. Also sometimes I cant open ( or preview ) pictures from iMessage or email. But as soon as I copy it ( on Desktop or folder ) it opens. It is really annoying. After the last restart - it worked for some time and I thought it is fixed. But now it is still the same. Any ideas anyone ?