Hello,
thank you very much for your help.
Meanwhile I have created some code, that ensures that the user can only upload images of type PNG. But the problem here is that I use the data type Blob as field and if you click on the field in the page to upload an image, the default logic comes here. As soon as one apparently deposits a field of the type Blob and of the subtype Bitmap, standard functions are always run through. Only after I have then uploaded an image via the standard, next comes my window with the upload only for PNG images. Can I switch off the standard logic somehow? My logic is always later...my code is passed through, in the OnValidate trigger of the new field "Image".