Change a filename from picture.png to picture.jpg and the name looks different immediately. The image data inside has not automatically become JPEG data. A filename is a label; conversion is a process that reads one representation and writes another.
Three pieces of information can disagree
A local file has a name and actual contents. On the web, the server can also send a media type in the Content-Type header. MDN explains that browsers use media types to decide how to process resources, rather than simply relying on the extension in the address.
This is why a renamed file might fail in one application and appear to work in another. An application may recognise the contents despite the misleading name. That does not mean the rename performed a conversion.
Use an export, then check the output
For an image, open a copy in an appropriate editor and choose an export format that meets the destination’s requirements. Check the resulting dimensions, transparency and appearance. Keep the working original separate so the export is not your only usable version.
For example, a transparent logo and a photograph may need different handling. Choosing a format is part of the decision; changing the letters after a dot is not a substitute. Our image-format guide explains how to start with the task.
Watch the whole filename
Some interfaces hide extensions, making a file’s visible name incomplete. Before troubleshooting, inspect its properties or enable the relevant filename display option for your system. Record what the application reports as well as what the file is called.
If a website rejects an upload, note its stated formats and size limit. A correctly converted file can still be too large or otherwise unsupported. Diagnose the specific requirement instead of repeatedly renaming the same data.