To use this script, you need to set assoc and ftype (Run cmd.exe as Administrator!) as given here:

assoc .session=SciTE4AutoIt3.File.session

and...

for compiled AutoIt script ...
ftype SciTE4AutoIt3.File.session="F:\AutoIt\AutoIt3_Tools\Load_SciTE_Session\Load_SciTE_Session.exe" "%1"

for uncompiled AutoIt script ...
ftype SciTE4AutoIt3.File.session="C:\Program Files (x86)\AutoIt3\AutoIt3.exe" "F:\AutoIt\AutoIt3_Tools\Load_SciTE_Session\Load_SciTE_Session.au3" "%1"

undo the changes ...
assoc .session=SciTE.session
ftype SciTE4AutoIt3.File.session=
ftype SciTE.session="F:\AutoIt\AutoIt3_Tools\Load_SciTE_Session\SciTE\SciTE.exe" "%1"

or reset the assoc for .session ... .session is usually not set!
assoc .session=
ftype SciTE4AutoIt3.File.session=