when you open a new case in the Partner Portal, in the bottom of the Case page, there's the following Note:
Note: To add a file, please create the case first, then come back to the case through "My Support Cases" and use the Attach a File Button.
In other words, you have to open the case, then access the Case page and upload the file.
If you have problems (I had!), add a comment to your case and the Splunk Support will send you a dropbox to upload the file; in this case remember to open another case for this problem!