"Invalid Form Data" in file upload on slack API - swift

Question

I'm working on a project where I need to upload files to channels in a slack workspace using slack's API. This is how I'm reading the file contents:
func readFile(localFilePath: String) -> Data? {
var contents: Data
let url = URL(fileURLWithPath: localFilePath)
do {
contents = try Data(contentsOf: url)
} catch {
return nil
}
return contents
}
However, this is what I get on slack:
I have also tried reading the file as a String, like this:
contents = try String(contentsOf: url, encoding: .ascii)
This way slack appears to recognize it's a PDF, but it's blank (see image below).
If I add an explicit Content-Type header and set it to multipart/form-data;, I get an invalid_form_data error in response.
Here's the request code:
let reqHeaders: HTTPHeaders = [
"Authorization": "",
"Accept": "application/json",
"Content-Type": "multipart/form-data;",
]
let params: [String:Any] = [
"channels": channelSelections.joined(separator: ","),
"content": fileData,
"initial_comment": messageText,
"filename": filename
]
AF.request(url, method: .post, parameters: params, headers: reqHeaders).responseJSON { response in
switch response.result {
case .success:
if let data = response.data {
do {
let json = try JSON(data: data)
if json["ok"].boolValue {
// some code
} else {
print("\n\n\n\(json)\n\n\n")
}
}
catch{
print("JSON Error")
}
}
case let .failure(error):
print(error)
}
}
What's the correct way to read the file and send as payload to slack's API?