The proper article for the filename "StrangerThingsS04E051080pHindiEnglishEsubsThemoviesflixmkv" is .

Because the title starts with the word "Stranger," which begins with a consonant sound, you would use "a" if it were an indefinite noun. However, since this filename refers to a specific, unique episode of a television series, the definite article is the most grammatically appropriate choice (e.g., "The StrangerThingsS04E05... file").