FIX: Standardised lowercase VFX asset extension import

[SamChenYu]

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Purpose of this PR

VFX Graph file importing is not standardised to be lowercase across the package, causing edge cases where case sensitivity is required. This PR fixes it.
Steps to repro:

1. Create a VFX Block Subgraph
2. Create another VFX Block Subgraph and change the extension to capitialised
3. The editor will import them differently:

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Testing status

Local Testing

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Comments to reviewers

Nil.