Fix media import uploads for ETL file types

[apodacaduron]

Description

Adds the source: "import" field to media import chunk upload requests while preserving Dropzone’s chunk metadata.

Without the import source flag, the backend validates uploads against MEDIA_ALLOWED_EXTENSIONS instead of ETL_VID_EXT, causing allowed media import file types like .mov to fail with 415.

How to Test

1. Go to /import/media/.
2. Click “Upload files”.
3. Select a file type allowed by ETL_VID_EXT but not MEDIA_ALLOWED_EXTENSIONS, such as .mov.
4. Confirm the upload succeeds and does not return 415.
5. In DevTools Network, confirm /admin/api/media/chunk includes source: import plus chunk fields like dzuuid and dzchunkindex.
6. Confirm normal actor/bulletin media uploads still validate against MEDIA_ALLOWED_EXTENSIONS.

Jira ID (if applicable)

BYNT-1695

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✨ Finishing Touches

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[apodacaduron]

Closing this PR as #334 solves it in a much simpler way