Portfolio "export whole discussion" button should not be visible if the user has inadequate permissions

When portfolios are enabled on the forum, there is a button to "Export whole discussion to portfolio". If the user is a student with default permissions, clicking on this button will result in the following being displayed:

Sorry but you do not have the required permissions to export files from this area

Really this button should not be visible if the user does not have adequate permissions.

Edit by kosswa:
Steps to reproduce

Prerequisites: Course with teacher T and student S enrolled.

1. Login as admin
2. Enable setting enableportfolios
3. Go to Site administration > Plugins > Portfolios > Manage portfolios and set the value for the portfolio plugin File download to Enabled and visible
4. Logout
5. Login as as teacher T
6. Go to the course, turn editing mode on and add a forum to the course
7. Create a new topic
8. Open this topic and click on the button "Export whole discussion to portfolio"
9. See that a new export page is showed
10. Logout
11. Login as student S
12. Go to the course
13. Open the forum
14. Open the topic
15. Click on the button "Export whole discussion to portfolio"
16. Then you should see the error message "Sorry but you do not have the required permissions to export files from this area"

Error Message

Debug info: Error code: nopermissions

Stack trace: line 188 of /portfolio/add.php: portfolio_caller_exception thrown

Expected Behavior
The button should not be visible to users without the capability mod/forum:exportdiscussion