GIMP not exporting personal channel

Question

I have created an image with a parchment-like effect. I achieved that creating a personal channel and applying to it the filter Render > Noise > Solid noise. I then painted on different layers what I needed without problems.

The problem came out when I exported the image. The output file doesn't keep the filter and I only get a plain color:

I exported the image in some of the most used formats (png, jpg, pdf, gif) and edited the exporting options without results.

How can I export my file keeping the parchment effect?

If more informations are needed please ask for them and consider that I don't know much about the graphics design world.

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Update 1

I'm adding some informations as required by @xenoid. To keep it simple, the following screenshots are from a "MWE" image with the same settings as the original one - which is rather complex - that behaves the same.

The following screenshots (click to enlarge) are, from the left to the right, the layers list, what GIMP shows to me and what I get when I export the image.

I confirm that the image is color-indexed and that I export it through File > Export... (or File > Export As...).

Update 2

Image converted to RGB mode (Image > Mode > RGB). Something better - now I see the overlay layer - but I still don't have the noise channel.

Update 3

"MWE" image available here.

Answer 1

I had a look at your XCF file.

PNG and JPEG don't support additional channels. They are RGBa (4 channels) for PNG, and RGB (3 channels) for JPEG. Any additional channels you add will be completely ignored on export to these formats.

To add a texture to an existing image, don't add it on a channel. Instead add it on a new layer, and then change the blending mode to Multiply, and perhaps reduce the opacity of the layer a little.

Example: