Append timestamp to file names for OSM Files

[mberg]

Currently, OMK assumes OSM ids are unique. If this is not the case, the osm submission will overwrite an existing OSM file (with the same name) on an ODK compatible server.

ODK appends a time stamp to all media attachments to avoid this. We should consider doing this to OSM files. This is particularly important if you want to generate OSM files from a shapefile (ie - you don't have time to get the data into OSM first or it's not appropriate to add to OSM).

• ensures the file being uploaded is unique as possible, this will prevent duplicates on the server side
• conform to how odk collect handles attachments

Note: If we do this we'll need to probably strip the timestamp from the OSM files before submitting back into OSM as updates.

[mberg]

To support this we could add and optional flag for support for stripping timestamps when exporting osm files