Change timestamp column in Batch Attachment Upload Dialog

[bronwyncombs]

Batch Attachment Upload Data Sets dialog lists
Name | Timestamp Created | Timestamp Modified

This matches the Queries dialog, rather than WorkBench or Batch Edit which also have the upload function.

With its current setup, you can only see if an attachment data set has been uploaded by clicking the Pencil and checking for an "Uploaded" field in the metadata:

Timestamp Modified should be changed to Timestamp Uploaded to match the WorkBench and Batch Edit data set dialogs. This makes more sense for the resource type shown in this dialog. Timestamp Modified will still be stored in the metadata for when users might need to know this.