B1G Harvest: exclude .zip files from harvest

[ewlarson]

Some local thumbnail solr field data points to .zip files:

[4607b35a-0b2e-498a-869f-677e84538a21] [remote_content_type] application/zip
[4607b35a-0b2e-498a-869f-677e84538a21] [viewer_protocol] map
[4607b35a-0b2e-498a-869f-677e84538a21] [image_url] http://data.pgc.umn.edu/maps/antarctica/navoceano/09/geo/Bellingshausen%20Sea.zip
[4607b35a-0b2e-498a-869f-677e84538a21] [gblsi_thumbnail_uri] http://data.pgc.umn.edu/maps/antarctica/navoceano/09/geo/Bellingshausen%20Sea.zip
[4607b35a-0b2e-498a-869f-677e84538a21] [service_url] 
[4607b35a-0b2e-498a-869f-677e84538a21] [image_extension] .zip
[4607b35a-0b2e-498a-869f-677e84538a21] [exception] #<ActiveRecord::RecordInvalid: Validation failed: Image You are not allowed to upload "zip" files, allowed types: jpg, jpeg, gif, png>

Bypass these requests, since they won't produce a valid thumb to upload.

[ewlarson]

Really appears to be all thumbs collected from "http://data.pgc.umn.edu"