default chunk_size in upload_attachment is wrong

[bctcvai]

In tator-py, the default chunk size for uploading attachments is way too big. It should be set to 10 megabytes or less.

https://github.com/cvisionai/tator-py/blob/d8460889a29e265acbb30b1006e60b66d326d835/tator/util/upload_attachment.py#L5

Will add commit to 1.3.8