Preserve extension of uploaded file

Currently a file extension is not saved when uploading a file. It should be preserved and set correctly when downloading a file.

Use getOriginalFilename in https://github.com/openmrs/openmrs-module-webservices.rest/blob/master/omod-1.8/src/main/java/org/openmrs/module/webservices/rest/web/v1_0/resource/openmrs1_8/ObsResource1_8.java#L439

https://talk.openmrs.org/t/how-to-set-file-name-and-or-extension-in-rest-with-complex-obs/9676