[Product-Developers] Not allowed to upload a file of this type to this folder

[Maarten Nieber]

Hi,

my Dexterity content type has a rich text description that can be edited with 
TinyMCE (the usual case). When I press the Image button in the editor and try to 
upload a new jpg image, I get the message

"Not allowed to upload a file of this type to this folder"

I have my verbose-security flag activated.
Does anyone know which setting is preventing me from uploading the jpg? 

Thanks!
Maarten