What is the combined size of the files that you try to upload? It should not be larger than the default limit set by ASP.NET which is 4 mb. In order to increase that limit you must do some tweaks to the web.config. Please, take a look at the following help-article
for details. You might also need to increase the executionTimeout property of the httpRunime section in the web.config. More information regarding the httpRuntime section can be found here.
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