docs: clarify MIME type changes when migrating from Express 4 to 5

[achaljhawar]

This PR improves the Express 5 migration guide by documenting an important change in MIME type handling that affects users migrating from Express 4 to Express 5.

Issue #1880

When migrating from Express 4 to Express 5, we discovered that JavaScript files served via res.sendFile() now return a Content-Type of "text/javascript" instead of the previous "application/javascript". This change occurs because Express 5 switched from its internal MIME type handling to the mime-types package.

Changes

• Added explanatory notes to the express.static.mime section explaining that this change affects other methods like res.sendFile()
• Added a specific note to the res.sendfile() section warning about potential Content-Type differences
• Provided concrete examples of the JavaScript MIME type change to help users anticipate similar issues

These documentation improvements will help users understand and prepare for potential MIME type differences when upgrading to Express 5, even if they weren't directly using express.static.mime.

Developer's Certificate of Origin 1.1 is acknowledged and agreed to.

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[achaljhawar]

@bjohansebas I have added it back

[achaljhawar]

@bjohansebas I have done the changes you asked me to do.

[ShubhamOulkar]

Thank you @achaljhawar