Resistor placement in filter circuits
Some things in nature simply aren't commutative.  A R-L filter is one of them.

In this case, the signal must pass thru one of the components, then the result is shunted to ground by the other.  It should be obvious that if you make the signal pass thru a high resistance then shunt the result to ground with a low resistance, you don't get much.  Conversely, if the signal passes thru a low resistance then is shunted to ground thru a high resistance, it will be largely unaffected.

In this case, you can think of the inductor as a frequency-variable resistor.  That's not exactly what it is, but it's a good enough mental model to understand low pass and high pass L-R filters at this level for now.

The effective resistance of the inductor goes up with frequency.  If it is in series, it will cause high frequencies to be attenuated more.  If it is the shunt, then it will affect high frequencies less.

Low pass filter:



High pass filter: