I have a question for you :)
Is there any way, to upload file (image/css file/js file) on jira application server (either to installation_dir or jiraHome_dir) and after access it via some sort of link? (like use it in 'Text' gadget via html tag <img src="link_to_uploaded_element">)
any other way than by web server*
You can make it work, but the instant you do it, you have rendered your Jira unsupported.
As @Fadoua suggests, if you could tell us what the real goal is, we can probably give you some guidance on a better way. Without the context, my best guess is that you really should be writing an app to contain your page(s) and having Jira serve them up correctly (with that approach, while your app is not supported by Atlassian, you can turn it off in the UI and become supported across the rest of Jira easily)
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