Difference between notify/reindexObject and "Save" [RESOLVED]

Hi,
I try to show a custom content-type with restapi.
Image comes from an Postgresql-Database as base64 encoded string.
And I think (nearly) everything works (image sizes generation, view,...)

But If I curl the object with restapi, I get following error:

No converter for making <plone.namedfile.file.NamedBlobImage object at 0x115ca26e0> (<class 'plone.namedfile.file.NamedBlobImage'>) JSON compatible.

After saving the obect via Edit-Tab, curl also works as it should...

I already do the following in my script:

notify(ObjectModifiedEvent(existing_obj))
existing_obj.reindexObject()

EDIT: this Topic can be deleted - I mixed Byte and Base64 encoded data...