Actually, how do you upload your file through the system? Are you using the File Manager or the Images & Documents module? If latter is the case, the problem might hide somewhere in the settings of the module. Either way, we would need some more information about your problem. How do you upload the file, do you have similar problems with other file formats or only for this extension? Sending the Sitefinity.log file, located in the App_Data folder will be of help too.
Thanks in advance for your cooperation.
All the best,
the Telerik team