Hi Saturnio,
Honestly, without more insight in the code you uploaded (in case it differs from the tutorial, even slightly), what's currently in your blobstore, the exact line where it bugs, if the code is exactly as the tutorial or was tweaked, the full stack trace, etc. This is really rough to answer.
Anyway, normally, when the question is a one off "this doesn't work", we actually suggest our customers to go on Stack Overflow with such a question. I would give
this link (or
this one) a read to make sure your question will be well received there, and ask it on Stack. We try to keep Google Groups for discussions (best practices, answers that require a lot more work than Stack would normally allow), and we also have our issue tracker (
here) to report defect or ask for feature requests.
We do monitor stack overflow for our tags :) So if the community doesn't answer, you will still get our help.
Cheers!