How works to PIN or not to PIN an optional bigfile video when streamed?

### Step 1: Please describe your environment

* ZeroNet version: 0.6.3 rev3502
* Operating system: W10
* Web browser: FFbeta
* Tor status: available
* Opened port: no

### Step 2: Describe the problem:

Hello, it seems that on a zeronet based media streaming site the optional bigfile was PINned (making it undeletable by the "garbage collector") just because it was a few seconds played. The size on drive appears to be several hundred of megabytes even it was watched several seconds and closed as not worth to be watched/shared. Though the file is PINned :-/ [Here are](http://127.0.0.1:43110/Talk.ZeroNetwork.bit/?Topic:1530097903_1NZftAz9VSawGF5JgMYGFv4ByoKEaQbNZu#comment_168_1NZftAz9VSawGF5JgMYGFv4ByoKEaQbNZu) more details. Is there anything that can be done so potentialy unwanted bigfiles are not automaticaly PINned and do not use large amount of data on HDD even minority of the file was downloaded?

No source file or test is named. Start by reproducing the partial video playback in the reported ZeroNet 0.6.3 environment and trace how streamed optional bigfiles become PINned. Done means an unwanted partially watched bigfile is not retained permanently and remains eligible for garbage collection.

索引モデルが issue の本文から書いたものです。