WIP: Feedback attachment

https://github.com/geoadmin/mf-geoadmin3/issues/1522

Current implementation accepts only one attachment.
Similarly to the Import KML, this feature is not available in IE9.

[Test Link] (http://mf-geoadmin3.dev.bgdi.ch/gal_feedback_attachement/src/?X=202235.27&Y=752102.91&zoom=1&lang=en&topic=ech&bgLayer=ch.swisstopo.pixelkarte-farbe)


Probably renaming the functions to make it clearer that they validate, but also show an alert if something is wrong? (`validateFormat` and `validateSize`)