Adding FileWillBeUploaded plugin hook (#9169)

* Adding file upload hook.

* Adding hook test for FileWillBeUploaded

* Some debugging fixes.

* Fix typo.

* Fixing double close

* Fix capitalization on docs.

plugin/hooks.go

@@ -4,6 +4,7 @@
 package plugin
 
 import (
+	"io"
 	"net/http"
 
 	"github.com/mattermost/mattermost-server/model"
@@ -28,6 +29,7 @@ const (
 	UserHasJoinedTeamId     = 11
 	UserHasLeftTeamId       = 12
 	ChannelHasBeenCreatedId = 13
+	FileWillBeUploadedId    = 14
 	TotalHooksId            = iota
 )
 
@@ -113,4 +115,11 @@ type Hooks interface {
 	// UserHasLeftTeam is invoked after the membership has been removed from the database.
 	// If actor is not nil, the user was removed from the team by the actor.
 	UserHasLeftTeam(c *Context, teamMember *model.TeamMember, actor *model.User)
+
+	// FileWillBeUploaded is invoked when a file is uploaded, but before it is committed to backing store.
+	// Read from file to retrieve the body of the uploaded file. You may modify the body of the file by writing to output.
+	// Returned FileInfo will be used instead of input FileInfo. Return nil to reject the file upload and include a text reason as the second argument.
+	// Note that this method will be called for files uploaded by plugins, including the plugin that uploaded the post.
+	// FileInfo.Size will be automatically set properly if you modify the file.
+	FileWillBeUploaded(c *Context, info *model.FileInfo, file io.Reader, output io.Writer) (*model.FileInfo, string)
 }