Append string to all the files inside a directory excluding subdirectories and .zip files

Hii,

Could someone help me to append string to the starting of all the filenames inside a directory but it should exclude .zip files and subdirectories.

Eg.
file1: test1.log
file2: test2.log
file3 test.zip
After running the script
file1: string_test1.log
file2: string_test2.log
file3: test.zip

Thanks in advance,