Fix blueprints zip uploads in the new add site design

[katinthehatsite]

Related issues

I spotted this while working on something else.

How AI was used in this PR

It was used to identify the root cause.

Proposed Changes

This PR ensures that you can upload bundled zip blueprints through the Upload blueprint file in the new add site flow similarly to how it worked in the old add site flow.

Testing Instructions

• Pull the changes from this branch
• Start Studio with npm start
• Enable the Enable Blueprints Gallery feature flag on the top
• Click on Add site > Build a new site
• Use the Upload blueprints button
• Ensure that you can uploaded a blueprints zip e.g. wedding-rsvp.zip

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 8699861 vs trunk

app-size

Metric	trunk	8699861	Diff	Change
App Size (Mac)	1409.87 MB	1409.87 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	8699861	Diff	Change
load	1506 ms	1493 ms	13 ms	⚪ 0.0%

site-startup

Metric	trunk	8699861	Diff	Change
siteCreation	8089 ms	8083 ms	6 ms	⚪ 0.0%
siteStartup	4937 ms	4941 ms	+4 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)