Unable to set Customer Portal logo

summary

It's not possible to upload a custom logo in the Customer Portal Look & Feel setting.

steps to reproduce

1. go to Customer Portal > Look & Feel;
2. select Customize look & feel of this Customer Portal;
3. click Choose logo and select a file to be uploaded;
4. the image will be displayed in the page as expected;
5. click Save.

Expected result: new Customer Portal logo is saved.
Actual result: a message about a successful update is displayed, but the image is discarded and the screen gets back to the Use default look & feel for this Customer Portal option.