Detect and set proper Content-Type for avatars

[paskal]

Detect proper avatar type to return instead of returning image/*, which is not valid.

Remark42 , before:

~ ❯ make rundev
~ ❯ curl -sI -X GET http://127.0.0.1:8083/api/v1/avatar/02625d3d98771929a8a6b1ff4c30628d66389ad6.image | grep Content-Type
Content-Type: image/*

Remark42, after:

~ ❯ cd backend
~ ❯ go get github.com/go-pkgz/auth@23da433 ; go mod vendor
go: downloading github.com/go-pkgz/auth v1.24.2-0.20240921020949-23da433bf4bb
go: upgraded github.com/go-pkgz/auth v1.24.2-0.20240921014908-825b117d9ed9 => v1.24.2-0.20240921020949-23da433bf4bb
~ ❯ cd ..
~ ❯ make rundev
~ ❯ curl -sI -X GET http://127.0.0.1:8083/api/v1/avatar/02625d3d98771929a8a6b1ff4c30628d66389ad6.image | grep Content-Type
Content-Type: image/png

[coveralls]

Pull Request Test Coverage Report for Build 10983518138

Details

• 9 of 16 (56.25%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 83.285%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
avatar/avatar.go	9	16	56.25%
<!--	Total:	9	16	56.25%	-->

Files with Coverage Reduction	New Missed Lines	%
avatar/avatar.go	1	80.23%
<!--	Total:	1		-->

Totals
Change from base Build 10950583576:	-0.2%
Covered Lines:	2591
Relevant Lines:	3111

---

💛 - Coveralls