After typing ip-address/developer unable to open index.htm . It is showing"index.htm not found"

From the first I was not able to upload my file, it is also showing that “file extension shouldnot exceed 3 character”

Make ABSOLUTELY sure that the file you upload is named index.htm and nothing else.
I positively guarantee you if you use anything else, it won’t work.

1st, this thing which you are doing i.e. ip address/developer is for uploading the index.htm file to your bolt, so you need to create an index.htm file with the code given in the module 2/LED
2nd check that you save the file with an extension .htm and not as a .txt file with a name index.htm
3rd you need to go to bolt ip address/developer and click on upload button, browse for your file and upload it.

*if the page bolt_ip_address/developer does not open then there may be a problem with your bolt(highly unlikely) or you have entered the ip address of your bilt wrong