Fix potential crash in ChunkedFile (#38)

Convert forced type cast to a conditional type cast and throw error when the latter fails

Author: andrewjl-mux

Sources/MuxUploadSDK/InternalUtilities/ChunkedFile.swift

@@ -46,7 +46,9 @@ class ChunkedFile {
     public func openFile(fileURL: URL) throws {
         if fileHandle == nil {
             do {
-                let fileSize = try readFileSize(url: fileURL)
+                guard let fileSize = try FileManager.default.attributesOfItem(atPath: fileURL.path)[FileAttributeKey.size] as? UInt64 else {
+                    throw ChunkedFileError.invalidState("Cannot retrieve file size")
+                }
                 self._fileSize = fileSize
 
                 let handle = try FileHandle(forReadingFrom: fileURL)
@@ -103,11 +105,6 @@ class ChunkedFile {
         return chunk
     }
 
-    private func readFileSize(url: URL) throws -> UInt64 {
-        let fileAttr = try FileManager.default.attributesOfItem(atPath: url.path)
-        return fileAttr[FileAttributeKey.size] as! UInt64
-    }
-    
     /// Creates a ``ChunkedFile`` that wraps the file given by the URL. The file will be opened after  calling ``openFile()``
     init(chunkSize: Int) {
         self.chunkSize = chunkSize