fix: APP-316 decoding file URLs

[blushi]

Description

Since I've merged https://github.com/regen-network/regen-server/pull/483, there was an issue when uploading data post file and previewing them on the form because we were not using the decoded URL.

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Author Checklist

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I have...

• provided a link to the relevant issue or specification
• provided instructions on how to test
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

How to test

Create a post and upload a file, if that's an image you should see the preview in the "edit file" form, then after submitting you should see the file preview in the list of files on the create post form.

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• confirmed all author checklist items have been addressed
• reviewed code correctness and readability
• verified React components follow DRY principles
• reviewed documentation is accurate
• reviewed tests
• manually tested (if applicable)

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