We are downloading items from Portal (zip files of replicas). It would seem from testing that the download runs for 100 seconds and then just moves on. A stream is returned, however it is corrupt because it is not the complete file stream. No exceptions are returned to the calling method until you attempt to use the stream.
We tried adjusting the httpRuntime tag in web.config in the portal folder in IIS thinking that might have some control
<httpRuntime maxUrlLength="2000" maxRequestLength="2097151" executionTimeout="100000" targetFramework="4.5" />
<httpRuntime maxUrlLength="2000" maxRequestLength="2097151" executionTimeout="200000" targetFramework="4.5" />
Which had no impact. It seems like there is something in the internal HttpClient call in the API that stops at 100 seconds. We have remote users downloading large file which we expect would take more than 100 seconds so it is pretty critical to be able to extend out this time.