The extension is a label

The characters after the last dot tell the system which program to open it with. Renaming song.mp3 to song.jpg does not turn it into a picture. It just makes everything confused.

The ones you meet constantly

• Images-.jpg photos, .png transparency, .webp smaller and modern, .svg vector and infinitely scalable
• Documents-.pdf looks the same everywhere, .docx editable
• Code-.html, .css, .js, .php, all plain text
• Archives-.zip many files in one

Turn on file extensions

Windows hides them by default. That is how invoice.pdf.exe appears as invoice.pdf and gets double-clicked. In File Explorer: View → Show → File name extensions. Do it now.

SVG for logos, always

A vector file stays sharp at any size and is usually a few kilobytes. A logo saved as a 2MB PNG and scaled down is both blurry and slow.

PDF when it must not change

Send a quotation as a PDF. Sending a .docx means the recipient's fonts, spacing and page breaks may differ, and they can edit it.