Preview consumes all Browser Resources

[cocheok]

Hi, the library some problems when you try to preview large size videos. The solution that i found is to put some restriction on fileinput.js like this:

  ```
    if(self.maxFilePreviewSize != undefined && file.size > self.maxFilePreviewSize) {
                self._previewDefault(file, previewId);
                self._initFileActions();
            }

maxFilePreviewSize is a new input parameter that informs the maximum size that a preview could have, in the case that the maxFilePreviewSize is smaller than the file size it shows the image.

I insert it in 

                ```
readFile = function (i) { 
                            ....
                            ....
                            if (!self.showPreview) {
                                   self.addToStack(file);
                                   setTimeout(readFile(i + 1), 100);
                                   self._raise('fileloaded', [file, previewId, i, reader]);
                                   return;
                              }
                             My solution
                             ...
                             ....
                         }

Please ulpload this change.