Problem

Unable to upload .JPG files when Attachment Setting Filter only allows .JPG files.

Attachment Setting
Attach File in Issue
File Extension

Solution

Use a file third-party online converter

Root Cause

When you change a file extension within the Window Explorer file browser/Finder Window, it does not change the file's type but causes the computer to misidentify the file into performing how software programs would open such file type.

How to Check File's MIME Type

Open Terminal/Command Prompt, and navigate to the file's directory
Key in the following command in the command line: file --mime-type -b filename

Result:

Akeles-MacBook-Pro:~ akeles$ file --mime-type -b AttachmentChecker_Logo_144x144.jpg
image/png
Akeles-MacBook-Pro:~ akeles$ file --mime-type -b AkelesBlockLogo.jpg
image/jpeg

Although AttachmentChecker_Logo_144x144.jpg has a file extension .JPG, it's MIME type is image/png. Where as AkelesBlockLogo.jpg has a MIME type image/jpeg.