Page caching breaks file upload progress bar for anonymous users

To reproduce the bug:

1. Add a File field called "File" to the Article content type
2. Set the field's "Progress indicator" to "Bar with progress meter"
3. Grant the "Article: Create new content" permission to the "Anonymous user" role
4. Enable page caching for anonymous users
5. Log out from the site
6. Go to node/add/article
7. Browse for a large enough file in the File field and click "Upload"

Expected behavior: Moving progress bar reflecting the upload's progress.

Observed behavior: Progress bar stuck at 0% with the "Starting upload..." message displayed until the file is fully uploaded.

Proposed solution: Firebug reveals that requests to the file/progress Ajax callback are being cached when page caching is turned on for anonymous users. This causes the callback to always return the "Starting upload..." message with -1 for the upload progress. The attached patch solves the issue by marking the file/progress Ajax callback uncacheable.