feat(builders): detect file MIME types from extension instead of hardcoding

[SAY-5]

Description

Adds a TelegramEx.MimeType helper that resolves a file's content type from its extension, and uses it in the Photo, Video, Sticker, and Document builders instead of hardcoding the MIME type. Unknown extensions fall back to application/octet-stream.

Ready Checklist

• Implementation: code works, types are specified, dialyzer passes
• Documentation: functions have @doc, README updated if needed
• Tests: tests added/updated, mix test passes locally
• Example: usage example added to /example

Related Issues

Closes #10

[lsdrfrx]

Thank you for your first PR, @SAY-5! Glad to see a new contributor!