At the bottom you can find the button + Add Files. Click on it to be prompted to choose the file you want to upload. Here is an example:
After you selected the file and entered all the relevant details into the form, scroll to the bottom to find the button Submit My Case. Click this button to submit your ticket including the attached files.
Case already created
Then, select My Cases within the drop-down selection.
On this new page, click on the Case Number where you would like to attach files. A new page will open and will contain all information related to your support case.
To attach documents from this page, click on + Add Files and choose the file you want to upload.
Finally, click on Submit Attachments to submit your attached files.
Please note that if you receive a message stating that the file exceeds the maximum file size (5 MB), try to compress the file and upload the *.zip archive you created.