Update Attachment class to ps standards

[firstred]

Questions	Answers
Branch?	develop
Description?	Upgrading the PHPDoc comments of this class, using the latest PS standards and also applying the PS standards to some of the code. Also getting the Franglais out of the comments, getting rid of things the Symfony code sniffer nags about, etc. It might have some additional todo tags, depending on whether the code was comprehensible or not.
Type?	improvement
Category?	CO
BC breaks?	no
Deprecations?	no
Fixed ticket?	N/A
How to test?	N/A

Warning: this PR also has a code cleanup. Please check this first before merging!

[mickaelandrieu]

Thanks !