MM-43828: Pass object length for some image operations (#20711)

For user profile and plugin upload, we use a bytes.Buffer.
In that case, we know the object size and can find
it out from the length of the buffer.

This helps reduce multi-part uploads.

This approach can also be taken in thumbnail and preview
images. However, they use an io.Pipe to directly upload
the image as it is being encoded. We could make the whole
process in separate parts of writing the full image in the
buffer and then upload it. But taking a conservative approach
for now.

Also, while here, removed some unused code.

https://mattermost.atlassian.net/browse/MM-43828

```release-note
NONE
```

app/plugin_hooks_test.go

@@ -462,15 +462,12 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 		}, th.App, func(*model.Manifest) plugin.API { return &mockAPI })
 		defer tearDown()
 
-		_, err := th.App.UploadFiles(th.Context,
-			"noteam",
+		_, err := th.App.UploadFile(th.Context,
+			[]byte("inputfile"),
 			th.BasicChannel.Id,
-			th.BasicUser.Id,
-			[]io.ReadCloser{ioutil.NopCloser(bytes.NewBufferString("inputfile"))},
-			[]string{"testhook.txt"},
-			[]string{},
-			time.Now(),
+			"testhook.txt",
 		)
+
 		if assert.NotNil(t, err) {
 			assert.Equal(t, "File rejected by plugin. rejected", err.Message)
 		}
@@ -515,15 +512,12 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 		}, th.App, func(*model.Manifest) plugin.API { return &mockAPI })
 		defer tearDown()
 
-		_, err := th.App.UploadFiles(th.Context,
-			"noteam",
+		_, err := th.App.UploadFile(th.Context,
+			[]byte("inputfile"),
 			th.BasicChannel.Id,
-			th.BasicUser.Id,
-			[]io.ReadCloser{ioutil.NopCloser(bytes.NewBufferString("inputfile"))},
-			[]string{"testhook.txt"},
-			[]string{},
-			time.Now(),
+			"testhook.txt",
 		)
+
 		if assert.NotNil(t, err) {
 			assert.Equal(t, "File rejected by plugin. rejected", err.Message)
 		}
@@ -562,20 +556,16 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 		}, th.App, func(*model.Manifest) plugin.API { return &mockAPI })
 		defer tearDown()
 
-		response, err := th.App.UploadFiles(th.Context,
-			"noteam",
+		response, err := th.App.UploadFile(th.Context,
+			[]byte("inputfile"),
 			th.BasicChannel.Id,
-			th.BasicUser.Id,
-			[]io.ReadCloser{ioutil.NopCloser(bytes.NewBufferString("inputfile"))},
-			[]string{"testhook.txt"},
-			[]string{},
-			time.Now(),
+			"testhook.txt",
 		)
+
 		assert.Nil(t, err)
 		assert.NotNil(t, response)
-		assert.Equal(t, 1, len(response.FileInfos))
 
-		fileID := response.FileInfos[0].Id
+		fileID := response.Id
 		fileInfo, err := th.App.GetFileInfo(fileID)
 		assert.Nil(t, err)
 		assert.NotNil(t, fileInfo)
@@ -638,19 +628,14 @@ func TestHookFileWillBeUploaded(t *testing.T) {
 		}, th.App, func(*model.Manifest) plugin.API { return &mockAPI })
 		defer tearDown()
 
-		response, err := th.App.UploadFiles(th.Context,
-			"noteam",
+		response, err := th.App.UploadFile(th.Context,
+			[]byte("inputfile"),
 			th.BasicChannel.Id,
-			th.BasicUser.Id,
-			[]io.ReadCloser{ioutil.NopCloser(bytes.NewBufferString("inputfile"))},
-			[]string{"testhook.txt"},
-			[]string{},
-			time.Now(),
+			"testhook.txt",
 		)
 		assert.Nil(t, err)
 		assert.NotNil(t, response)
-		assert.Equal(t, 1, len(response.FileInfos))
-		fileID := response.FileInfos[0].Id
+		fileID := response.Id
 
 		fileInfo, err := th.App.GetFileInfo(fileID)
 		assert.Nil(t, err)