From OpenSCADAWiki
Currently there implemented only requesting files by the GET request of HTTP. There are supported also the Partial Content requests in single range and with forcing to this mode at some configured file size. 
That is work properly only for video and audio files and doesn't work for other files especially documents, so you have to configure the file size limit for solid reading in maximum size of your documents. For proper working the Partial Content requests we need to implement the HTTP request HEAD in the module Protocol.HTTP before implementing here!