[common][storage] Ensure get_file_content returns what is expected

The method is written and documented to return the file
content as a string. However it's also used to read the
content of C++ binary blob.

Therefore, a parameter has been added to be able to
select opening mode and by default it's text.

beat/web/common/storage.py

@@ -61,7 +61,7 @@ class OverwriteStorage(FileSystemStorage):
 # ----------------------------------------------------------
 
 
-def get_file_content(instance, attr):
+def get_file_content(instance, attr, binary=False):
     """Reads the contents of a Django-FileField attribute
 
     Parameters:
@@ -81,8 +81,12 @@ def get_file_content(instance, attr):
     if f is None:
         return ""
 
+    mode = "rt"
+    if binary:
+        mode = "rb"
+
     try:
-        f.open()
+        f.open(mode)
     except Exception:
         return ""