Refactor mime type

[Prashidha Kharel (cool8137)]

This PR refactors the mime type definition to make it easy to extend mime type in the future.

Problem

The image and file mime types were originally hard coded strings defined at different places. Extending supported filetype would require changes to be made at many places.

Solution

This PR centralises the definition of mime type by doing the following:

• Define mime types in src/lib/multimodal-utils.ts as an enum like object MIME_TYPE.
• Create define type guards for SupportedImageType and SupportedFileype.
• Use the mime type object values and type guards whenever checking for mime type.
• Add unit tests using vitest.

Testing

Two Unit test files have been added for the multimodal-utils and multipodal preview fiels.

Here is a screen shot of local usage of image upload which is working as expected.

I was inspired for this refactor when I was trying to add a new mime type for my personal project.

Although this PR is not linked to any existing issue, I hope it is small and useful enough to merge.

Let me know if you have any comments.

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