Chuck, I can explane now.
There is no need for uploading a clean file at all.
This has to do with translations. Our browser (IE8 Dutch) has the possibillity to translate any language into Dutch. We had the traslation on and started change awnsers. After saving it happened. Original uploaded photo filesizes did not appear anymore and you could see what happened in the adminpart of photopost
This morning we replaced the awnsers written in Dutch back into English, saved again and everyting was as it used tu be, including the filesizes.
This is the most easy and solid solution to the problem I discribed. Maybe you can keep this in mind when others are having the same or simular problem.
The advise is: Don't change any awnsers while translation is running and if you already had, replace the awnsers from own language into English and save.
Glad to be helpful in this matter.