So, I just tested leaving the allowed filetypes-field blank (ergo allowing everything) and that worked. The mov-file gets uploaded flawlessly.
But interestingly the response is:name: "sample.mov" path: "/opt/bitnami/apache2/htdocs/public/uploads/files/1496310135527-sample.qt" url: "/assets/uploads/files/1496310135527-sample.qt"
Adding just the mov extension to the allowed file-extensions blocks it again.
Now the response suggests to me somehow, that nodebb is converting the uploaded file to a .qt-file.
So I just added the qt-extension to the allowed files and it works.
It's really weird behavior, but I found a solution. Thanks for participating everyone!