Selenium: after file upload can not get file size - selenium

Question

I use Selenium to test web app behaviour on file upload page.
The goal of test is to check application validations on file selection.
I use webDriver.SendKeys (C#) on file input to simulate file upload, file is selected successfully, but fileInput.files[0].size (js) is always 0 (or undefined?) and app shows error "Do not upload empty files". File size is correct on manual testing.
Any suggestions, on how to solve the problem?
UPDATE Code details:
HTML
<input id="file-path" type="file" data-bind="attr: { accept: mimeTypes }, event: { change: onFileSelect }" />
<small id="file-path-error" class="field-validation-error" data-bind="validationMessage: file"></small>
JS (file edit model uses knockout)
this.file = ko.observable().extend({
validation: [
{
validator: function(file) {
// valid if no file or file size < 5Mb
return !file || file.size < 5242880;
},
message: 'File size should be less than 5Mb'
}, {
validator: function(file) {
// valid if no file or file size > 0
return !file || file.size > 0;
},
message: 'Do not upload empty files'
}
]
});
this.onFileSelect = function(model, event) {
var fileInput;
fileInput = event.target;
if ((fileInput.files != null) && fileInput.files.length > 0) {
_this.file(fileInput.files[0]);
} else {
_this.file(null);
}
};
C#
webDriver.FindElement(By.Id("file-path")).SendKeys("C:\testFile.txt");
Assert.AreEqual(webDriver.FindElement(By.Id("file-path-error").Text,"");
Assertion always fails because file-path-error contains "Do not upload empty files" - that is the problem.

Answer 1

I was misleaded by answers on How to automate file upload in Internet Explorer using Selenium?
Discusstion on How to set a value to a file input in HTML? says that actually we can not upload files programmatically.
So there was no sence in expecting decent behaviour for such test.