* Add new Attachment model fields:
- is_image
- thumbnail
* Cache if the attachment is an image
* Add new setting for controlling max upload size
* Validate uploaded attachment file
* Add tqdm for progress bars
* Refactor migrations
- Don't need is_image field
- Can introspect from the thumbnail
* Data migration for existing attachments
* Bump API version
* Update tests and validators
* Add "is_image" field to the Attachment model
* Offload to background task
* Implement unit tests
* Docs
* Add unit test for data migration
* Additional unit test
* Omit migration tests from code coverage
* Additional unit tests
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