In WCF.Attachment._upload: Check mime type before passing file to Resizer.js

[TimWolla]

Currently files are unconditionally passed to Resizer.js, leading to bogus error messages, because resizer.loadFile fails:

WCF/wcfsetup/install/files/js/WCF.Attachment.js

Line 339 in 0ae6b18

var promise = resizer.loadFile(file)

[WCF.Attachment] Failed to resize image "foo.pdf":
error { target: img, isTrusted: true, srcElement: img, currentTarget: img, eventPhase: 2, bubbles: false, cancelable: false, returnValue: true, defaultPrevented: false, composed: false, … }

Ideally instead the mime type should be checked to be something resizable and the complete resizing logic skipped if it is not.

see also: #2770