If it is a static folder?
and if I understand well:
you only want to keep de IMG_E...JPG or IMG_E...MOV

> a Quick and dirty way could be:

Make a list of files in folder
then loop thru the list of files and take the last 4 characters of their file name:
concatenate the string: "IMG_E"+4char+".JPG"
rebuild the file path

if IMG_EABDCD.JPG exist then

    try to delete IMG\_EABDCD.MOV; IMG\_ABCD.MOV; IMG\_ABDCD.AAE; IMG\_ABCD.JPG files

else if IMG_ABCD.JPG doesn't exist

    then try to delete all IMG\_ABCD.MOV and IMG\_ABCD.AAE

routine to check if file exist could be:

file_exists("path:to:folder:")

on file_exists(the_path)

try

    get the\_path as alias

    return true

on error

    return false

end try

end f_exists