Hi,
I think you current up-loader is some third party ajax uploader. So first we need to check its configuration because client might be facing issue on specific files and not all files.
In order to check the issue i need access to your developer workstation.
About me:
My name is Tahir; I'm a Microsoft Certified Solutions Developer(MCSD) for web located in Islamabad. I've a sound experience in c#, vb.net, asp.net, mvc, entity frame work, sql server and bootstrap.
I'm highly interested in your work, and i believe it fits my area of expertise.
I'm looking forward to hear from you.