"farm_boy-win32-x64.7z" (If you pronounce it as the word "farm" )
While the name starts with the letter "f," the grammatical rule for choosing between "a" and "an" depends on the of the first letter. In English, the letter "f" is pronounced starting with a vowel sound ( "ef" ). farm_boy-win32-x64.7z
Since "farm" is a common word, most people will read the filename starting with a consonant "f" sound, making the natural choice. "farm_boy-win32-x64
The proper article to use before is "a." farm_boy-win32-x64.7z
"farm_boy-win32-x64.7z" (If you are spelling out the letters: "ef-ay-ar-em..." )