If writing a file we always want Create

This will truncate existing files.

Fixes fyne-io#1168

CHANGELOG.md

@@ -34,6 +34,7 @@ More detailed release notes can be found on the [releases page](https://github.c
 * Empty main menu causes panic (#1073)
 * Installing using `fyne install` on Linux now works on distrubutions that don't use `/usr/local`
 * Fix recommendations from staticcheck
+* Unable to overwrite file when using dialog.ShowFileSave (#1168)
 
 
 ## 1.3 - 5 June 2020

internal/driver/glfw/file.go

@@ -36,10 +36,7 @@ func openFile(uri fyne.URI, create bool) (*file, error) {
 	}
 
 	path := uri.String()[7:]
-	f, err := os.Open(path)
-	if err != nil && create {
-		f, err = os.Create(path)
-	}
+	f, err := os.Create(path) // If it exists this will truncate which is what we wanted
 
 	return &file{File: f, path: path}, err
 }