The name of the folder you store your extension files in is not really relevant. The root folder of the extension is the folder where you have your qext file.
if you have a qext file called xxxx.qext and store it in C:\Users\<User>\Documents\Qlik\Sense\Extensions\yyyy where you also have a xxxx.js, the path to xxxx.js will be something like http://localhost:4848/extensions/xxxx/xxxx.js.
Erik Wetterberg https://extendingqlik.upper88.com/
a) Why is your local wbfolder.wbl dated from august ? Workbench editor uses this file to display all extension files in different tabs. Can you edit it to see what is inside (the file list is in plain text) ?
b) Try to import a new file in workbench, i.e. a png image. Then check if it appears in your local file directory.
If it doesn't, let's search for it in all extensions, may be you have a name conflict with another "heatmap" qext definition ? Has this new file name been added to wbfolder.wbl ?
Then activate dev tools CTRL/Shift J. In the Sources tab, expand the "localhost:4848" item. Then expand "Extension/HeatMap" folder. You shouls see your HeatMap.js? followed by some digits. Select this file, right clic on "Copy link address" and paste elsewhere to check the complete path of this file; or right clic on "Open link in new tab" to check the actual script content.
An advice : QS Help state that "The Qext file name is used as identification for the visualization which means that, especially if you intend to make it available for others to use, it must be a unique file name." I strongly advice you to give your Qext file a name including your company and domain, ie com-acme-heatmap.qext if you work at Acme. Same rule should be applied to your main js file.