Buffering river but it came out as circle

Something has gone wrong with the georeferencing of the initial map you used. When I load your files into qgis and use the measurement tool, the river is 0.12 meters from end to end. I am afraid you need to redigitize it using a better map. The quickest way is probably to install the Open Layers plugin, then you can use Web -> Open Layers to load open street map or a number of other global background map.

I can make a guess at what has happened. Did you "set" the coordinate system of the river to UTM 50N or did you actually reproject it? If the original line was digitized over a map in Long/Lat WGS84, you cannot just set the coordinate system to something else. The line coordinates are actually in Long/Lat degrees, not UTM.

I reset the River layer back to WGS84 (EPSG:4326) then I did a buffer at distance 0.0003 (approx = 30 meters, in degrees) and the result looks like:

What I suggest is:

1. Set the River layer back to it's correct WGS84 EPSG:4326 coordinate system
2. Reproject ("Export -> Save Layer As...") to the desired UTM 50 coordinate system. This creates a new, reprojected layer
3. Do the 30 meter buffer on this newly projected river line.