file size limits for image attachments

[justschen]

the GH limit for images atm is 5mb per image.

we'll maybe set a limit of 30mb per image since we are scaling images down anyways, and scaled images according to https://platform.openai.com/docs/guides/vision#calculating-costs brings our image sizes to between 0.2mb and 1mb.

explore main reason for slowdowns when attaching large images: likely a mix of hashing and the attempt to scale the image.