sniff magic bytes for application/octet-stream to select correct previewer

[badnikhil]

PR Description

This PR sniffs the first bytes of the response body using lookupMimeType from the mime package (already a transitive dependency via dio and share_plus) to detect the true format and route it to the correct previewer. The mime package is also added as a direct dependency to make the usage explicit.

The sniffing only applies when the declared type is exactly application/octet-stream and falls back gracefully if the format cannot be identified.
Sniffing is kept in the Previewer widget rather than the model layer because the sniffed type is a local rendering hint only, and i think moving it to the model would create an inconsistency between the persisted headers (which correctly reflect what the server sent) and the mediaType getter.

Related Issues

• Closes Previewer fallback based on magic bytes #1382

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• Yes
• No, and this is why: The fix delegates entirely to mime's lookupMimeType which is well-tested upstream.Also , the approach first needs to be reviewed.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux