I have a c# .Net 4.5 SOAP web service hosted in IIS 8.5. As this service takes hits I've seen it constantly writing .tmp and .post files to this directory
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\webservices_itcrateengine\2c7c6f10\8cbc7b94\uploads.
Files are uploaded in MIME multipart/form-data format. By default, all requests, including form fields and uploaded files, larger than 256 KB are buffered to disk, rather than held in server memory.
also regarding the buffer
The amount of data that is buffered in server memory for a request, which includes file uploads, can be specified by accessing the RequestLengthDiskThreshold property or by setting the requestLengthDiskThreshold attribute of the httpRuntime Element (ASP.NET Settings Schema) element within the Machine.config or Web.config file.
By adjusting the MaxRequestLength and RequestLengthDiskThreshold properties, you can fine tune the performance of your server
So you can have a higher setting in your web.config e.g. 1024 Kb
<httpRuntime requestLengthDiskThreshold = "1024"
Specifies the limit for the input stream buffering threshold, in kilobytes. Also This value should not exceed the maxRequestLength attribute.