add missing fixture to ApplicationHelperTest

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013172
use with_settings at AttachmentsControllerTest#test_show_text_file_should_send_if_too_big

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013173
add test of attachment download etag

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013174