If you are using IIS for hosting your application, then the default upload file size if 4MB. To increase it, please use this below section in your web.config -

<configuration>
    <system.web>
        <httpRuntime maxRequestLength="1048576" />
    </system.web>
</configuration>

For IIS7 and above, you also need to add the lines below:

<system.webServer>
   <security>
      <requestFiltering>
         <requestLimits maxAllowedContentLength="1073741824" />
      </requestFiltering>
   </security>
</system.webServer>

Note: maxAllowedContentLength is measured in bytes while maxRequestLength is measured in kilobytes, which is why the values differ in this config example. (Both are equivalent to 1 GB.)