Reducing the filestore dependencies from the rest of the source code (#16816)

* Reducing the filestore dependencies from the rest of the source code

* Making more generic config conversion to FileBackendSettings

* Fixing usage of the NewFileBackend function

* Fixing more usages of the NewFileBackend function

* Fix some linter errors

* Fix more linter errors

* Fixing some unit tests

* Fixing linter problem

* Addressing PR review comments

* Simplifing the CopyFile for tests

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/file.go

@@ -12,11 +12,6 @@ const (
 	MaxImageSize = int64(6048 * 4032) // 24 megapixels, roughly 36MB as a raw image
 )
 
-var (
-	IMAGE_EXTENSIONS = [7]string{".jpg", ".jpeg", ".gif", ".bmp", ".png", ".tiff", "tif"}
-	IMAGE_MIME_TYPES = map[string]string{".jpg": "image/jpeg", ".jpeg": "image/jpeg", ".gif": "image/gif", ".bmp": "image/bmp", ".png": "image/png", ".tiff": "image/tiff", ".tif": "image/tif"}
-)
-
 type FileUploadResponse struct {
 	FileInfos []*FileInfo `json:"file_infos"`
 	ClientIds []string    `json:"client_ids"`