remove exe from testdata (#561)

After considering #550 and #551, there seems it's hard to create a
windows executable that satisfies these 2 conditions:
- is small in size
- does not trigger antivirus alerts; seems like many antiviruses just
  don't care what's inside an exe. If it's .exe then it's a virus.

Looking back on it, adding fixture files is not perfect: seems nice to
have the library tests work with real files, but:
- it does not count towards test-coverage
- real files increase repo size

Going forward I think I will remove more and more of the files in
testdata folder, and add more unit tests.

mimetype_test.go

@@ -59,7 +59,6 @@ var files = map[string]string{
 	"dwg.dwg":            "image/vnd.dwg",
 	"eot.eot":            "application/vnd.ms-fontobject",
 	"epub.epub":          "application/epub+zip",
-	"exe.exe":            "application/vnd.microsoft.portable-executable",
 	"fdf.fdf":            "application/vnd.fdf",
 	"fits.fits":          "application/fits",
 	"flac.flac":          "audio/flac",