@gervin. Thanks for your question. Let me see if I can explain. What you specify in the module is the MIME type. This MIME type is an Internet standard that defines or specifies the file format. If you look at a list of mime types, you will see that jpg, jpeg, and jfif all have the same mime type: image/jpeg. The mime type is used to verify that the file being uploaded is truly a jpeg or jpg or jfif file. It is used for security.
To upload a JPG file, just select it in the file browser when you click browse. It will upload it.
So, in summary, the mime type image/jpeg refers to .JPG, .JPEG, or .JFIF files. Hope this helps.