faster uploads by avoiding unneed large blob reads

[djay]

replaces #155

fixed #167

As mentioned in the comments perhaps a better long term fix is to change zope.schema to avoid accessing properies if they are decorators and instead using something like - https://stackoverflow.com/questions/16169948/check-if-something-is-an-attribute-or-decorator-in-python first

[mister-roboto]

@djay thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[djay]

I created a ticket for zope.schema suggestion. zopefoundation/zope.schema#127