after experiencing this issue myself - i foudn that i needed to add the Mitel IP address to your browser's "trusted sites" list.
to do this :
- in IE, select Tools, Internet options
- Click the "security" tab
- select "trusted sites"
- select "sites" button
- enter the Mitel IP address if it's not there already (i.e.
https://192.168.1.2)
- select "Add"
exit out of all of your dialog boxes, log out of the mitel system and shut down all IE windows. Once you log back in, you should be able to upload the file.
this worked for me.