Variable not initialized

In my Logic App I used a custom function named AppendBlob. To this function I wanted to pass a function header named BlobName. I tried to set BlobName from a variable, but then I received the following failure:
The variable ‘File’ must be initialized before it can be used inside action ‘XXX’.

“variables”: {
“File”: “filename.txt”

“AppendBlobCreate”: {

“headers”: {
“BlobName”: “@variables(‘File’)”,

The value of variable File is a hard-coded value. To initiate the variable, you will have  to use square brackets instead of the @ symbol. Just a hint to keep you from searching your ass off. I don’t know why it works like that.
“BlobName”: “[variables(‘File’)]”