Merge branch '5.3'

diff --cc wcfsetup/install/files/js/WCF.Attachment.js
index 4ced08677cbcf123a84059edeb803c4dfef5f3cb,34ff910301e24eeb3062ebc3edacb9e14a3fb0bf..a00269e6b4329bb54d01f02f4a5a852b7e889e1a
--- 1/wcfsetup/install/files/js/WCF.Attachment.js
--- 2/wcfsetup/install/files/js/WCF.Attachment.js
+++ b/wcfsetup/install/files/js/WCF.Attachment.js
@@@ -335,8 -335,9 +335,9 @@@ WCF.Attachment.Upload = WCF.Upload.exte
                                // As our resizer is based on Pica it will use multiple workers per image if possible.
                                promise = Array.prototype.reduce.call(files, (function (acc, file) {
                                        return acc.then((function (arr) {
-                                               // Ignore anything that is not one of the 4 basic image types.
-                                               if (['image/png', 'image/gif', 'image/jpeg', 'image/webp'].indexOf(file.type) === -1) {
 -                                              // Ignore anything that is neither PNG nor JPEG. GIFs are not supported
 -                                              // due to the support for animations.
 -                                              if (['image/png', 'image/jpeg'].indexOf(file.type) === -1) {
++                                              // Ignore anything that is not a widely used mimetype for static images.
++                                              // GIFs are not supported due to the support for animations.
++                                              if (['image/png', 'image/jpeg', 'image/webp'].indexOf(file.type) === -1) {
                                                        arr.push(file);
                                                        return arr;
                                                }