Weird file type/name?

".tmp" = it is a "temp" or temporary file.

When digital video is captured, a temporary file is started and the digital video information is stored there - temporarily. When the capture is complete, the temporary file receives the "end-of file activity" trigger and all the data in the temp file turns into the video file - in this case "VID_20180626_161008.mp4". The application which captured the file put a default name on it (in this case, "VID_20180626_161008" which is the date of the recording (June 26 2018) and a tracking number ("161008") so it is not confused with other captured video files and the .MP4 extension tells us what video file format is storing the video.

After "Stop" is pressed on the video capture, that temp file will be emptied of data - there is nothing useful in there for Windows (or Android or macOS or any other operating system). If you want the video file, you need "VID_20180626_161008.mp4".

From my perspective, what you report is expected behavior.

you didn't stop the recording...the TMP extension means that it is not a usable file yet, that it is still being written to. check again in 5 mins, or hit the STOP button, NOT the pause button.

work out,
what format you recorded it in,
then try simply changing the file extension name to that.

tmp, is short for temporary. it might have been the transition file you are trying to view.
search elsewhere for the complete file.

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.tmp usually means that it's a temporary file, typically what the system makes while it is recording a specific file before it saves the actual recording, so it may not actually be a complete file.

You can try deleting the .tmp extension and leaving the mp4 but you probably need to make sure you actually saved the file properly on your phone next time.