This is the process I use to see which extensions may be blocked for upload forms.
This process uses Burp Suite
Process
Create a wordlist with extensions
Launch Burp and make sure it is set to intercept all browser traffic
Next upload a file to the form. Any file works for this
Send to Intruder
Click on the Positions and select the Sniper option
Find the filename and Add the Add button next to it. This positions where you are going to attack
Click Payloads, add the file that you created with the extensions
Start Attack