Naive fix for improperly formatted image input.

[Symbiomatrix]

Description

Please include:

• Motivation: Can't override an uploadable image + sketch via a component; after 2-3 calls the image turns into a base64 str in preprocess and fails the assertion until it is emptied.
• Summary: Wraps base64 string (no validation) as an {image, mask}. Mask is copied from image currently. Might be preferable to set it to some blank value, but in base64 it's a drag. None causes an error down the line, in mask_im; I'd alter this as well but consequences are unclear.
• Fixes: Cannot override upload image + sketch, NoneType attributeerror. #4088 .
• Dependencies: None.

Closes: #4088

Checklist:

• I have performed a self-review of my own code
• I have added a short summary of my change to the CHANGELOG.md
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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[abidlabs]

This is not an appropriate fix. We need to understand why the data is being sent in a string format from the frontend instead of a dictionary format. @pngwn is looking into this issue, so I'll close this PR