invalid character '<' looking for beginning of value

Hey there
Thanks for this plugin! it is awsome for private projects with a not so big infrastructure to back things up :D
I wanted to build a arch package on my server with drone and send it to me via telegram, but the documents are not send, there is just the error
```
2018/02/23 22:19:15 invalid character '<' looking for beginning of value
```
my drone.yml is like this

```
pipeline:
build:
image: lerentis/build:arch
commands:
- "pacman -S --needed base-devel git wget yajl glibc glib2 icu libbsd libedit libutil-linux libxml2 ncurses python2 sqlite cmake ninja perl python2-requests python2-six python2-sphinx rsync swig clang --noconfirm"
- "git clone https://aur.archlinux.org/libblocksruntime.git"
- "chown -R nobody: libblocksruntime"
- "chmod -R g+ws libblocksruntime"
- "cd libblocksruntime"
- "sudo -u nobody makepkg"
- "pacman -U libblocksruntime-0.4.1-1-x86_64.pkg.tar.xz --noconfirm && cd .."
- "git clone https://aur.archlinux.org/swift-language.git"
- "chown -R nobody: swift-language"
- "chmod -R g+ws swift-language"
- "cd swift-language && sudo -u nobody makepkg"
- "ls -alh"
- "pwd"
- "mv swift-4.0.3-2-x86_64.pkg.tar.xz ../"
- "mv swift-lldb-4.0.3-2-x86_64.pkg.tar.xz ../"
cpu_quota: 0.80
when:
event: [ push, pull_request ]

telegram:
image: appleboy/drone-telegram
token: masked
to: masked
document:
- swift-4.0.3-2-x86_64.pkg.tar.xz
- swift-lldb-4.0.3-2-x86_64.pkg.tar.xz
```

is there any issue with adding archives with this plugin? I have tested to just send empty files with the same name like this `echo test > swift-4.0.3-2-x86_64.pkg.tar.xz` which sends the file, so i think it is not the name of the file.

Any help or hint in the right direction would be greatly appreciated

Reproduce the error using the supplied drone.yml, comparing archive uploads with the empty-file case. Trace the plugin's Telegram document-upload path and inspect the response that begins with '<'; done when both listed .pkg.tar.xz archives send successfully without the JSON parsing error.

Written by the indexing model from the issue text.