important fix: process uploads only if configuration allows it

[Markus-Gurkcity]

Questions	Answers
Description?	The PS configuration PS_CUSTOMER_SERVICE_FILE_UPLOAD only removes content from the template. there must be a validation in the process/send method if this configuration is enabled. See extended description and screenshots below...
Type?	bug fix / critical
BC breaks?	no
Deprecations?	no
Fixed ticket?	Fixes PrestaShop/Prestashop#{issue number here}.
How to test?	Please indicate how to best verify that this PR is correct.

Description:
in contactform module the PS configuration PS_CUSTOMER_SERVICE_FILE_UPLOAD is assigned to the template. If this configuration is false the upload file input and the form multipart attribute will be removed from the html:


The issue:
If a attacker modifies the html form in the browser and adds the form file upload input, the file will be send to prestashop an will be uploaded.

Fix:
check the configuration at the process/send method and reject any file uploads:

[ps-jarvis]

Hello @Markus-Gurkcity!

This is your first pull request on contactform repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!

[matthieu-rolland]

thank you @Markus-Gurkcity

@paulnoelcholot here is checking that the configuration allows file upload on the backend side, so that it's not possible to bypass this just by modifying the html in the browser

[paulnoelcholot]

QA by dev, since the How to Test is a bit hard to follow for a manual QA