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New features:
- Improve contenttype detection logic for unregistered but common types.
Change get_contenttype to support common types which are or were not registered
with IANA, like image/webp or audio/midi.
Note: image/webp is already a IANA registered type and also added by
Products.MimetypesRegistry.
[thet] (157-2)
- Support for allowed media types.
Support to constrain files to specific media types with a "accept" attribute on
file and image fields, just like the "accept" attribute of the HTML file input.
Fixes: 157
[thet] (157)