Update plugin.go

[theme1256]

Summary

Possible fix for issue #41.
Changing from markdown image to image-attatchment.
I've never written Go before and havn't tried the edit (because I don't have the tools), but VSCode can't find any syntax errors.

Ticket Link

Issue 41

[hanzei]

Could you please share a before and after screenshot?

[theme1256]

Here's a screenshot showing one message before change (the blank one) and another efter change (the working one)

[hanzei]

I'm hesitant to merge the PR as I don't full understand why the workaround is needed. Other plugin like https://github.com/moussetc/mattermost-plugin-giphy do use image and work on mobile.

[theme1256]

Sure, giphy works fine, but this plugin doesn't, despite both of them sending an image in markdown.

I did a bit of testing, manually sending images and gifs, and it turns out that gifs work just fine, but images don't.

So this PR is needed, because regular images don't work in the app when sendt in markdown, but fine when sendt as attatchments.

[theme1256]

Never mind.
It actually doesn't work.

I tried building and uploading the plugin to my own server. This code works, but the image still doesn't show. I guess this will have to be fixed in the actual app and not in this plugin.