Okay, I got complaints coming in that stuff broke for ppl who wanted to upload.
I now looked into it and also had initial issues sending my file to GDRIVE as I just passed the file object. Turned out the file object was an uppy-wrapped representation of the binary file looking like this.
In order to actually access the native file object, I had to work with files[0].data. (maybe a breaking change?)
Before that change, the handler just passed the native file object. I could expect other ppl to experience breaking functionality with this change, I am not sure.
Got everything working now. Thanks so much for the quick turnaround and support!