It does not matter to Rebex SFTP client or server what is the type of the file.
If one file is succesfully uploaded to your server and upload of another one fails, just let us know the exception stack trace that you get - e.g. the content of exception.ToString() along with log of the communication. The log can be created as described at http://www.rebex.net/kb/logging/
Either post the log here or send it to firstname.lastname@example.org. Seeing it we will be able to see what is going on and why the PutFile method was unable to transfer the file and help you solve this issue.