Thank you for your answer! I think I start to understand this now.
-----------------------------149589358933568048074206291858
Content-Disposition: form-data; name="type"
composer
-----------------------------149589358933568048074206291858
Content-Disposition: form-data; name="files[]"; filename="Screenshot_2020-05-05.png"
Content-Type: image/png
How can I reproduce this? I used the inspector and looked at the request and saw this when pasting a png, when I tried with a jpg it will say image/jpg. What does raw bitmap mean? With which browser does this happen (I use firefox)?
EDIT okay never mind, I think maybe I misunderstood what pasted means. I actually did drag and drop. I tried to copy and paste an image, but it seems like it does not work in general on my system. It just adds this text to the the composer:
x-special/nautilus-clipboard
copy
file:///home/username/Downloads/Screenshot_2020-05-05%20.png