safely handle large image files in blocks

There may be a problem with binary files being overwritten accidentally when saving gists via blockbuilder. a large image will be truncated and then loaded by the client. but if we try to save that file via the api it will break.
http://bl.ocks.org/Fil/c6efde7d8271a8728cd23c0d268ea787

an interesting note, small binary files don't seem to have this problem, so investigate with large image

Start by reproducing the behavior with the linked large image and compare it with a small binary file through the blockbuilder save-via-API path. Done means large binary files are saved without truncation or corruption while the existing small-binary behavior remains intact.

Written by the indexing model from the issue text.