Upload file working, but not showing in Edit page

I've inserted a field to upload a Image File. I've configured an upload folder with a relative path, and I've turned on the option "create thumbnails".

It's working to upload a file and to create thumbnail. I can delete the file too. And I can see the files via FTP.

But there is one strange thing: when I edit the record, the page shows this field with a default broken image icon.

I've tried to use absolute path and I've inserted the complete physical path, and it worked with the same problem.
Does anyone had this situation? Does anybody knows the solution?
Thanks in advance