Fix chat image upload double read

[sam-w]

Description

Fixes #3888

There is a bug which breaks image uploads in chats:

I believe this was caused by a change in 2ad86aa which causes an IO stream based on an image file to be read again without resetting the stream back to the beginning (first read in chat_backend.py, upload_files_for_chat, and then again in pg_file_store.py, create_populate_lobj). Non image files are treated differently and thus are unaffected.

How Has This Been Tested?

Tested both PDF and image uploading.

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