There seems to be about a 128 KB limit on the size of files that can be attached to posts. This is a small limit, smaller than it used to be.
When trying to attach a file larger than 128 KB, it seems to upload but then the status indicator shows an error (
), clicking on it gives a popup message
"ERROR Failed to move uploaded file."
it seems to be limiting it to the chunk size, whatever a chunk is in this case