You can not do it from the Web Connectors config page, but you can edit the URL to do this after you have copied and pasted it into your load script.
From the help document at:
You can attach up to 5 files to each email, totaling a maximum of 10MB. Include the following to the request:
fileAttachment[1-5]=[your local file path]
In QlikView you can modify the URL string on the fly, to add extra attachments or rename attachments. By default in Qlik Sense you have to have a Library which then can't be modified after creation. This is until you change the put the Server into Legacy mode. You can do this by going to the QMC, selecting Engines then editing your engine, under the Advanced section you need to un-tick Standard Mode and then select Apply.
Hope that helps.