In my case, I had to specify messageEncoding to Mtom in app.config of the client application like that:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.6.1" />
</startup>
<system.serviceModel>
<bindings>
<basicHttpBinding>
<binding name="IntegrationServiceSoap" messageEncoding="Mtom"/>
</basicHttpBinding>
</bindings>
<client>
<endpoint address="http://localhost:29495/IntegrationService.asmx"
binding="basicHttpBinding" bindingConfiguration="IntegrationServiceSoap"
contract="IntegrationService.IntegrationServiceSoap" name="IntegrationServiceSoap" />
</client>
</system.serviceModel>
</configuration>
Both my client and server use basicHttpBinding. I hope this helps the others :)