Unfortunately, Jade cannot extend templates. Instead you must include parts.

web/templates/404.jade

@@ -1,19 +0,0 @@
-extends base.jade
-
-block head
-	style(type="text/css").
-		#error {
-			font-size: 3.5rem;
-			line-height: 5rem;
-			text-align: center;
-			margin: 6rem 0; }
-
-		#error h1 { font-size: 8rem; }
-
-block header
-	h1 xDDDD
-
-block body
-	section#error
-		h1 404
-		| Nod Found xD

web/templates/base.jade

@@ -16,7 +16,7 @@ html(lang="en")
 				text.logo Arav's dwelling
 				text.under(y="11") Welcome to my sacred place, wanderer
 			nav
-				a(href=.MainSite title="Arav's dwelling") Back to main website
+				a(href=mainSite title="Arav's dwelling") Back to main website
 				block header
 					h1 Upload
 		block body

web/templates/base/body.footer.jade

@@ -0,0 +1,2 @@
+footer
+	| 2022 Arav <#[a(href="mailto:me@arav.top") me@arav.top]>

web/templates/base/body.header.jade

@@ -0,0 +1,7 @@
+body
+    header
+        svg#logo(viewBox="0 -25 216 40")
+            text.logo Arav's dwelling
+            text.under(y="11") Welcome to my sacred place, wanderer
+        nav
+            a(href=mainSite title="Arav's dwelling") Back to main website

web/templates/base/top.jade

@@ -0,0 +1,11 @@
+doctype html
+html(lang="en")
+	head
+		title Arav's dwelling / Upload
+		meta(charset="utf-8")
+		meta(http-equiv="X-UA-Compatible" content="IE=edge")
+		meta(name="viewport" content="width=device-width, initial-scale=1.0")
+		meta(name="theme-color" content="#cd2682")
+		meta(name="description" content="File upload service for ya.")
+		link(rel="icon" href="/assets/img/favicon.svg" sizes="any" type="image/svg+xml")
+		link(href="/assets/css/main.css" rel="stylesheet")

web/templates/error404.jade

@@ -0,0 +1,19 @@
+:go:func Error404(mainSite string)
+
+include base/top.jade
+	style(type="text/css").
+		#error {
+			font-size: 3.5rem;
+			line-height: 5rem;
+			text-align: center;
+			margin: 6rem 0; }
+
+		#error h1 { font-size: 8rem; }
+include base/body.header.jade
+	h1 404
+
+section#error
+	h1 404
+	| Nod Found xD
+
+include base/body.footer.jade

web/templates/index.jade

@@ -1,19 +1,23 @@
-extends base.jade
+:go:func Index(mainSite string, fileMaxSize, storageCapacity, storageUsed, keepForHours int, storageUsedStr, storageCapacityStr, storageAvailableStr string)
-block body
+
+include base/top.jade
+include base/body.header.jade
+	h1 Upload
+
 section#rules.center
 	h2 Rules
 	ul
-			li Max. file size is #{.FileMaxSz}.
+		li Max. file size is #{fileMaxSize}.
-			li It will be kept for #{.KeepForHours} hours.
+		li It will be kept for #{keepForHours} hours.
 		li Anything you upload should comply with Russian Federation's law. Generally speaking, anything illegal, like CP, extremist literature etc., is forbidden.
 section#used-space.center
 	h2 Free space
 	div
-			span #{.StorageUsedStr}
+		span #{storageUsedStr}
-			progress(value=.StorageUsed max=.StorageCapacity)
+		progress(value=storageUsed max=storageCapacity)
-			span #{.StorageCapStr}
+		span #{storageCapacityStr}
 	div
-			| #{.StorageAvailStr}
+		| #{storageAvailableStr}
 section#upload.center
 	h2 Upload
 	form(action="/" method="POST" enctype="multipart/form-data")
@@ -30,3 +34,5 @@ block body
 	p Upload logs include: access time, IP-address, name of a file it was uploaded with, a SHA-256 hash of the file, download name*, size of the file in bytes, User-Agent.
 	p Each download also being logged and include: access time, IP-address, name of a file it was requested with, download name*, User-Agent.
 	p *Download name is a salted SHA-256 hash of the file encoded using base64 raw URL encoding. The hash is being salted to make useless any attempt of bruteforcing.
+
+include base/body.footer.jade

web/templates/nospace.jade

@@ -1,6 +1,6 @@
-extends base.jade
+:go:func ErrorNoSpace(mainSite string)
 
-block head
+include base/top.jade
 	style(type="text/css").
 		#error {
 			font-size: 3.5rem;
@@ -9,10 +9,10 @@ block head
 			margin: 6rem 0; }
 
 		#error h1 { font-size: 6rem; }
-
+include base/body.header.jade
-block header
 	h1 :(
 
-block body
 section#error
 	h1 Not enough space left
+
+include base/body.footer.jade

web/templates/uploaded.jade

@@ -1,9 +1,15 @@
-extends base.jade
+:go:func Uploaded(mainSite, downloadLink string, keepForHours int)
-block body
+
+include base/top.jade
+include base/body.header.jade
+	h1 Upload
+
 section#file
 	h2 Your link
 	center
-			a(href=.DownloadURL) #{.DownloadURL}
+		a(href=downloadLink) #{downloadLink}
-		center It will be available for next #{.KeepForHours} hours.
+	center It will be available for next #{keepForHours} hours.
 	center
 		a(href="/") Back to index page
+
+include base/body.footer.jade