If someone else had this problem too, here is the fastest way to solve the problem:
Open the file. Use the "save as"-dialogue. Entering the new name, select the type
by ending option *.* below. Enter cpp as new ending.
Select the properties of the still open file, where you can see build, xx and advanced.
In the field advanced enter CPP instead of cc as language tag.
Use the add-file-to-project-option.
Use the remove-file-from-project-option with the old file.
You're done.