For some reason, i get error 404 on mediafiles in edit-mode. When viewing the site, an image got an url like "/globalassets/270x180_2.jpg", but in edit-mode the url is "/EPiServer/CMS/Content/globalassets/280x180_2.jpg,,41?epieditmode=False", which results in error 404. However, if i remove the last part ",,41?epieditmode=False" the it works. It's a newly installed 7,5 site, so what the heck did i do? :)
...and the solution is: Make sure to inherit from the correct classes, in this case ImageData NOT MediaData.