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 before converting.
If the message says unsupported file type or file format not supported, the fastest fix is usually to convert a copy into one of the accepted formats instead of renaming the file.
Best first fixes
Convert HEIC or WebP images to JPG, MOV or AVI videos to MP4, WAV or M4A audio to MP3, and office documents to PDF when a form rejects the original.
For Google Drive, school portals, job applications, and strict web forms, PDF, JPG, MP3, and MP4 are usually the safest first targets.
Do not convert blindly
Choose the format the destination asks for. A technically better format is not useful if the upload form refuses it.
Open the converted file before submitting so you catch rotated photos, missing pages, silent audio, video sync problems, or unreadable text.