Here is the list I had used twice

H1B1 extension:

Certified Form ETA 9035E
Filed Form I-129 and H Supplements
Explanation to the I-129 Petition (if you need to answer some questions in the form)
A copy of I-140/LC Petition (this is for over 6 years extension only)
Company’s letter of support of the H1B1 petition
Job deion and schedule for the extended years (for RFE only)
A copy of employment contract
A copy of Resume
Copies of I-797 Approval Notice and I-94
A copy of valid passport
Copies of diploma and tran with evaluation


The extension is not a big risk. Good luck!