Problem with uploading some files using Blueimp upload + cropper.js - php

Question

I have a system where users can send some photos to the server, and the client receive it to make polaroid versions of them.
The user has the option to crop them or not.
I used Blueimp (https://github.com/blueimp/jQuery-File-Upload) to upload the images and cropper.js (https://fengyuanchen.github.io/cropperjs/) to crop them.
But some users are complaining that some files is not uploading, but no error is shown.
Isn't the extension, isn't the size... I don't know what could be the problem...
In this file (jquery.fileupload-ui.js) I found a line of code that, if I remove it, it works, but I can't see the file's thumbnails. But the file is uploaded. Is the last line before the always function.
add: function (e, data) {
if (e.isDefaultPrevented()) {
return false;
}
var $this = $(this),
that = $this.data('blueimp-fileupload') ||
$this.data('fileupload'),
options = that.options;
data.context = that._renderUpload(data.files)
.data('data', data)
.addClass('processing');
options.filesContainer[
options.prependFiles ? 'prepend' : 'append'
](data.context);
that._forceReflow(data.context);
that._transition(data.context);
data.process(function () {
// return $this.fileupload('process', data);
}).always(function () {
data.context.each(function (index) {
$(this).find('.size').text(
that._formatFileSize(data.files[index].size)
);
}).removeClass('processing');
that._renderPreviews(data);
}).done(function () {
data.context.find('.start').prop('disabled', false);
if ((that._trigger('added', e, data) !== false) &&
(options.autoUpload || data.autoUpload) &&
data.autoUpload !== false) {
data.submit();
}
}).fail(function () {
if (data.files.error) {
data.context.each(function (index) {
var error = data.files[index].error;
if (error) {
$(this).find('.error').text(error);
}
});
}
});