Limit amount of data sent each time

[vitorbaptista]

As commented on #39 by @devanlai:

An easy way to trip this case is to run something that dumps lots of data, like dmesg.

It's also possible to avoid generating the 413 Request Entity Too Large error in the first place by limiting the amount of data read and sent in a single post request. For example, in stream_file:

max_post_size = 300 * 1024
# Url-encoding control characters = 3/1 overhead
# Base64 encoding = 4/3 overhead
# Worst case encoded size is 4x the original size
# Also allow some overhead for the other json fields
max_unencoded_size = (max_post_size // 4) - 64
data = f.read(max_unencoded_size)