日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

これは最後に書こう、なんて感じで置いといたことが多すぎたのか、やっぱり今日も長くなりそう。

In this sentence, how was 置く conjugated to 置いといた? I do realize it probably means the same as 置いていた, but I've never seen this grammar before, and IME tells me such a conjugation is correct.