I am currently exploring using the ExportMap function on the rest endpoint of a Cached Map Service.( I want to export only some layer within a Mapservice).
I have found that if you change the SupportedImageReturnTypes to MIME of a mxd based mapservice, you can export sublayers of a cached mapservice. (As an aside, you can't do this, if it is a msd based mapservice; even if you change SupportedImageReturnTypes from MIME+url to MIME, the exported image has all the layers. I think it directly returns the cached image.)
The ArcGIS Server help mentions that this setting
...Specifies whether images will be returned as MIME data or written to disk (MIME + URL). If you choose URL, you must have a server directory specified for the configuration.
I have tested this out and saw that if the setting was set to MIME+url, then a single temp png file is created in the output director, but it changes as the user pans the map & is deleted within a few seconds.
This is different from the cache, which remains on disk.
I am wondering if it is a good idea to set the setting to MIME, or would it have some consequence on performance.
Does anyone have any experience with this setting?