Read the accepted formats first
Most upload errors happen because the site accepts only a short list of extensions. Look for JPG, PNG, PDF, MP4, MP3, DOCX, or XLSX in the instructions.
Best first fixes
Convert HEIC or WebP images to JPG, MOV or AVI videos to MP4, WAV audio to MP3, and office documents to PDF when a form rejects the original.
Do not convert blindly
Choose the format the destination asks for. A technically better format is not useful if the upload form refuses it.