Charcoal, 81 bytes

≔Ｉ⪪Ｓ/θＷ﹪§θ±²§θ±¹⊞θι≧÷⌊θθ≔Ｘχ⌈Ｅ25⌕⮌Ｘ⁰↨§θ¹Ｉι⁰η¿﹪η§θ¹⪫…θ²/⁺÷§θ⁰§θ¹⭆Ｉ⁺η﹪קθ⁰÷η§θ¹η⎇κι. 

Try it online! Link is to verbose version of code. Explanation: This is the way overengineered version, which works with arbitrary precision positive integer inputs. I'll write a version golfed for the expected inputs later.

≔Ｉ⪪Ｓ/θ 

Input the fraction.

Ｗ﹪§θ±²§θ±¹⊞θι≧÷⌊θθ 

Reduce it to its lowest terms.

≔Ｘχ⌈Ｅ25⌕⮌Ｘ⁰↨§θ¹Ｉι⁰η 

Find a power of 10 that the denominator might be a factor of.

¿﹪η§θ¹⪫…θ²/ 

But if it isn't then just output the fraction.

⁺÷§θ⁰§θ¹⭆Ｉ⁺η﹪קθ⁰÷η§θ¹η⎇κι. 

Convert the fraction to arbitrary precision decimal.

Charcoal, 40 bytes

≔Ｉ⪪Ｓ/θ≧÷⌈Φ⊕⌊θ∧ι¬⌈﹪θιθ¿﹪Ｘχφ§θ¹⪫θ/Ｉ∕§θ⁰§θ¹ 

Try it online! Link is to verbose version of code. Explanation:

≔Ｉ⪪Ｓ/θ 

Input the fraction.

≧÷⌈Φ⊕⌊θ∧ι¬⌈﹪θιθ 

Reduce it to its lowest terms.

¿﹪Ｘχφ§θ¹ 

If the denominator is not a factor of 10¹⁰⁰⁰...

⪫θ/ 

... output as a fraction, otherwise...

Ｉ∕§θ⁰§θ¹ 

output as a decimal.

81 bytes for the way overengineered version which works with arbitrary precision positive integer inputs:

≔Ｉ⪪Ｓ/θＷ﹪§θ±²§θ±¹⊞θι≧÷⌊θθ≔Ｘχ⌈Ｅ25⌕⮌Ｘ⁰↨§θ¹Ｉι⁰η¿﹪η§θ¹⪫…θ²/⁺÷§θ⁰§θ¹⭆Ｉ⁺η﹪קθ⁰÷η§θ¹η⎇κι. 

Try it online! Link is to verbose version of code. Explanation:

≔Ｉ⪪Ｓ/θ 

Input the fraction.

Ｗ﹪§θ±²§θ±¹⊞θι≧÷⌊θθ 

Reduce it to its lowest terms.

≔Ｘχ⌈Ｅ25⌕⮌Ｘ⁰↨§θ¹Ｉι⁰η 

Find a power of 10 that the denominator might be a factor of.

¿﹪η§θ¹⪫…θ²/ 

But if it isn't then just output the fraction.

⁺÷§θ⁰§θ¹⭆Ｉ⁺η﹪קθ⁰÷η§θ¹η⎇κι. 

Convert the fraction to arbitrary precision decimal.