file.postedfile.saveas(path) gives error

Perhaps you have come across this one before.

I've loaded an image file using the fileupload method (VB code) into 'mainpicupload'
I've successfully checked it has something, that it is a .jpg and worked out the destination folder (~\uploaded_images)
calling the .saveas property gives me a
A first chance exception of type 'System.Web.HttpException' occurred in System.Web.dll

 
trying to set the mainpicupload.Filename to the new on is not allowed as it is readonly - so
Savepath = Savepath + destfilename.Value + LCase(Extension)
mainpicload.PostedFile.SaveAs(Savepath)
mainpicupload - the fileupload item
savepath - string containing '~\uploaded_images\n3115.jpg

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